@Solla-topee My answers in blue below.

- If possible, it is recommended to create a Guest WiFi  (an isolated network)on router, and hook one or two of the  KP200 to the Guest WiFi and see if plug behave the same

I wil try this, but it will take some time because this doesn't happen frequently, like i said it happens only once every couple weeks.

-Avoid put them on the same power strip, put them in different  locations and see if that helps. 

These are KP200 outlets.  They are not on a power strip, they are hard wired into the wall.  I don't know how we can put them in different locations.

-If you are using any third party software like Home automation, SmartThings Alexa, Google home apps etc., please unlink them temporarily and see if the issue occurs

We use Brilliant, but aren't using it to control the plugs, I will disconnect them from the Brilliant app.

-It is recommended to unplug the home appliance on the plug or try a different home appliance connected to the plug

We have two devices plugged into two different plugs.  As I mentioned, all 3 outlets turn off at the same time.

-Confirm Kasa app is up-to-date, and firmware of plugs is also up-to-date. 

Everything is up to date according to the Kasa App

I notice the camera is connected to the plug, may I know the model and power consumption of it？ 

It is a Nest Indoor camera power consumption is 0.2A

May I know do you have other plugs which work fine?

These are the only smart outlets we have we have one KP400 outdoor smart plug controlling outdoor lights, but we do not leave it on so it is not possible to tell if it experiences the same issue.

 May I know the model of your router and your network topology,do you use any smart home app, or other smart devices that use WiFi? 

I have a 3-node Linksys Velop Mesh network in bridge mode acting as an access point to the router which is an ASUS RT-AC3200.  The Linksys nodes are using a wired backhaul to ensure maximum signal strength.