I have had a similar experience with the HS-210's. Looking around for support information, this appears to be a "known" issue.

During the holidays, I had planned to install 6 of the HS-210's. The first two were on two different lighting circuits that only contained 3-way switches. The HS-210 was installed in the first switch location fed from the breaker. A "dumb" 3-way switch was left in the "load," or remote, location. These were installed successfully and per the circuit diagram shown above in this thread. I was able to control these with the Kasa app and Amazon Alexa.

The next lighting circuit included two 4-way switches wired between the 3-way switches. The HS-210, again, was installed in the first location fed from the breaker. And again, this installation was successful; full control and correct status with Kasa and Alexa.

After this, another lighting circuit with two 4-way switches, similar to the one mentioned in the paragraph above, was tried. Within seconds of the first operation, the Kasa app changed to an incorrect "off" status when the lights were on. The Kasa app was able to control but the issue with incorrect status continued. All switch locations could control the lights correctly, manually. This was also the case before adding the HS-210 switch.

The circuit was checked with a voltmeter. I was able to follow the traveler with nominal 120V through each switch. What I thought odd was the other traveler had a nominal 80V. (Sourced from the HS-210? This traveler was isolated from any other circuit except the HS-210.) The HS-210 was replaced with another HS-210 - same results.

After a discussion with tech support by phone, I was told that I had a wiring error. If the circuit diagram in this thread is correct, I believe that wiring is not an issue.

I found this thread today and tried changing the switch position of the "load" 3-way switch. This turned my lights on. I went to one of the two 4-way switch locations and turned the lights off. No other changes, wiring or otherwise, were made. The Kasa app now correctly indicates the on or off state of the lights when the app is used to control the lights. Alexa also is able to control the lights without issue. This is not a fix for the HS-210 issue. Obviously, there will be manual control of the swtiches that will change these switch positions in the future.

One other detail: The app is delayed in updating the true status of the lights by approximately 10-15 seconds. It would be very interesting to know how the HS-210 determines the actual status of the lights (current flow?). BTW, all lights are LED. The first successful 3-way/4-way circuit has nominally 40 watts. The circuit with issues is nominally 65 watts. 

Any ideas,thoughts, comments? How does the HS-210 sense light status?

@Adam1234 

I have the same exact issue - have you been able to resolve it?

@Adam1234 I have the exact same issue and it has frustrated me so much !!

One Kasa 3-way switch is working perfectly in one circuit, the second one in separate circuit is giving this issue. Did any one try to install 2 Smart switches ( HS210 on both ends ) in the circuit ?

@Francois62 

Unfortunately not. Tech support was basically useless. I tried everything I could think of. Currently just dealing with having to leave the "dumb" switch in the on position.

@Adam1234 I just ordered a 3-way switch of another brand from Amazon, will be delivered tomorrow. I will give it a shot and update.

@Adam1234 Very happy to report that less known branded switch worked perfectly, right out of the box, same wiring as TP-Link switch.

I went throguh lot of trouble with TP-link switch, first installed it on load side, and then on TP-Link's suggestion, installed it on Live side, even that didn't fixed the issue. Re-checked my wiring multiple times to ensure that I am not mixing any wires. This is very disappointing from TP-link perspective and deem it unreliable.

@Carl - How can I get replacement of the faulty switch ?. I bought Dual pack for HS210 from Amazon couple months ago, first worked fine and second one was giving issues as explained here.

@Adam1234 Hi Adam and all:

I have exact same problem as yours, I spent 3 hours chatting with TPLINK online chat tech people and get nothing useful.
 

I just found a way to solve this, replace both 3-way switches with KASA smart switch, it works for two of my light now, it doesn't matter how many 4 way in between or even no 4 way. And for some of you thought it works only replace one, it will only work when the other 3 way is on/off, not always, you can try.
 

Tech people asked you to reinstall the app, don't even listen, it is just waste of your time.

Just replace both of 3-way.
 

Hope it is useful
 

Thanks

Bolun

@Adam1234 

I have the exact same problem, I did the install both ways, with the line (hot) wire on the HS210 and the load on the regular 3way switch, and also having the regular switch on the line and the hs210 on the load side (see attached picture), and I had these problems: 

With the Kasa switch on the Line side and the 3 way switch down (off) 

-Can manually switch lights on and off on both sides

-Scheduled times in the app to turn the lights on or off don't work

-The lights indicator switch on for a moment then turned off even with the lights on and vice versa 

-The switch couldn't detect if the lights were on or off. 

-I can turn the lights on and off using alexa. 

With the Kasa switch on the Line side and the 3 way switch up (on)

all of the same I described before except: -

-Scheduled times in the app turn the lights on but they don't turn off at the scheduled time.

Now, 

With the kasa switch on the load side and the 3 way switch down (off) 

-Can manually switch lights on and off on both sides

-Scheduled times in the app turn the lights on but they don't turn off at the scheduled time.

-The lights status in the app turns on for a moment then goes off (doesn't matter if the light goes on or off)

-I can turn the lights on and off using alexa. 

With the kasa switch on the load side and the 3 way switch up (on) 

-Can manually switch lights on and off on both sides

-Scheduled times in the app can turn the lights on and off at the scheduled time.

-The lights indicator in the app shows if the lights are on or off correctly 

-I can turn the lights on and off using alexa. 

Hint: If the app cannot indicate properly if the lights are on or off, the other functions, scenes and schedules won't work properly. 

So, it seems that the only way this product works right is with the kasa switch on the load side and the regular 3 switch up (left wire hot) to achieve full lights control, which is disappointing because it should work right with the other 3 way switch up or down since power is still coming through one of the two travelers. 

It looks that by design the switch needs to be receiving or sending power through the left traveler wire (red) to work properly and its unable to do the same if the regular switch is down (power coming through the black traveler). 

I think this is a design failure or hardware limitation because the smart switch should be able to detect if the load is on or off on either of the two traveler wires and it looks like it can only do that through the left traveler, hence the need to have the other switch flipped up. So, in a way, its only sending dumb power through one of the two travelers, and can turn lights on but cannot turn them off because it doesn't sense if the appliance is lit or not, so this switch should only be installed on the load side with the regular switch flipped up.

Feit (not a product endorsement) has a smart dimmer that must be installed on the load side and uses the black traveler as a source of constant power to feed the smart switch, and the red wire is used only to signal (on-off) if the other switch was flipped and always work properly. I find this arrangement more convenient because this enables the smart switch to recognize when the load is on and if the other switch was flipped and properly control the lights. TP-link should have done the same.