I'm experiencing a similar problem using one HS210 in a 4-way configuration.

Symptoms of the problem: 

• Important: Most of the time everything (room lights, lighted ring on switch, Kasa app indicator ring, Alexa voice commands) work correctly. 
• Other times the switch turns on but the lighted switch ring will flicker on and off for 2 or 3 seconds and then correctly go out to indicate the room lights are on. 
• Other times the room light will turn on by voice command but not turn off by voice commands while the ring on the switch remains lighted but the room light is energized. 
• On other ocassions the indicator ring on the Kasa App will "spin" for 4 or 5 seconds and then indicate the light is on by turning green. 
• On ocassion, while using the Kasa App to control the lights, the ring in the app will turn green to indicate "on" and then turn gray after 2 - 3 seconds while the room light remains on.
• Sometimes the ring in the Kasa App will behave as described in the previous point but turn green again after 5 to 10 seconds without any intervention.
• All 3 wall switches in the circuit always work correctly manually. 

I have 5 other single Kasa 3-way switches in 4-way circuits and 1 in a 5-way and they all work as intended.  I am also not ruling out the access point that the switch is connected to.  The switch is showing 42dbm and that should be a strong enough signal. However, the switch is on a wall next to a double oven and a refrigerator.  The ovens and frig. are also between the switch and access point.  That could be a lot of sheet metal and/or intererence.  However, there is an adjacent (not in the same box) 3-way Kasa with the frig. and ovens between it and the access point that is not having any issues at all.

Remedies used: 

• When the switch is not working correctly it sometimes fixes itself after turning the other switches in the circuit on and off a couple of times. 
• Another time rebooting the router corrected the problem. 
• Finally, resetting the switch to factory default fixed the problem. 
• All remedies were temporary and there is no way to tell if a specific remedy was the action that corrected the problem.

@wbob Thank you, this is a good survey of the situation.  I am having a similar problem - one smart switch, one dumb switch, LED bulbs on or dim, never fully off; switch & software often confused as to the state of the system.  Preparing for installation, using an old-fashioned neon indicator, I definitely saw induced voltage on at least one of the travelers.  This traveler bundle runs from 2nd floor to attic and back down to ground floor, so it is pretty long.  Another setup in the house with both switches in the same small room works fine.  So, I suspected induced traveler voltage interacting with switch sensors and software not quite up to the task were causing the problems.  And most of what I have read here strengthens this suspicion.

@wbob For anyone interested:  The problem has been resolved by having the problematic switch connect to the main router rather than the access point.  While the signal from the access point was much stronger, by 20 dBm, than the main router, the operation of the switch has remained stable and trouble free for about a month.  I had a similar issue with a 3-way I installed in a garage.  It appears that interference and metal objects do cause problems with the connection.  There is a metal-sided cargo trailer in direct line of sight with the access point and the 3-way switch.  Again, good signal strenth did not equal good functionality.

wbob wrote

I'm experiencing a similar problem using one HS210 in a 4-way configuration.

 

Symptoms of the problem: 

• Important: Most of the time everything (room lights, lighted ring on switch, Kasa app indicator ring, Alexa voice commands) work correctly. 
• Other times the switch turns on but the lighted switch ring will flicker on and off for 2 or 3 seconds and then correctly go out to indicate the room lights are on. 
• Other times the room light will turn on by voice command but not turn off by voice commands while the ring on the switch remains lighted but the room light is energized. 
• On other ocassions the indicator ring on the Kasa App will "spin" for 4 or 5 seconds and then indicate the light is on by turning green. 
• On ocassion, while using the Kasa App to control the lights, the ring in the app will turn green to indicate "on" and then turn gray after 2 - 3 seconds while the room light remains on.
• Sometimes the ring in the Kasa App will behave as described in the previous point but turn green again after 5 to 10 seconds without any intervention.
• All 3 wall switches in the circuit always work correctly manually. 

 

I have 5 other single Kasa 3-way switches in 4-way circuits and 1 in a 5-way and they all work as intended.  I am also not ruling out the access point that the switch is connected to.  The switch is showing 42dbm and that should be a strong enough signal. However, the switch is on a wall next to a double oven and a refrigerator.  The ovens and frig. are also between the switch and access point.  That could be a lot of sheet metal and/or intererence.  However, there is an adjacent (not in the same box) 3-way Kasa with the frig. and ovens between it and the access point that is not having any issues at all.

 

Remedies used: 

• When the switch is not working correctly it sometimes fixes itself after turning the other switches in the circuit on and off a couple of times. 
• Another time rebooting the router corrected the problem. 
• Finally, resetting the switch to factory default fixed the problem. 
• All remedies were temporary and there is no way to tell if a specific remedy was the action that corrected the problem.

@wbob I just replaced the LED bulbs with incandescents.  Problem solved.  Probably the lower-resistance bulbs pull the induced voltage in the un-energized traveler down to something the switch recognizes as "off", and everyone is now happy.  Reading about your router issues, maybe there are two separate problems underlying these malfunctions.