I bought a couple of HS210 kits to replace a couple of 3-way switch in my house. I've now tried to install two different switches in two different 3-way runs and am getting the same behavior where they are flicking off and on when power is turned back on. It's gotten dark out so I've reverted to my dumb switches at this point with hopes of being able to try things out again in the morning if I can get some answers here. I'll provide some pictures below. 
 

Here's the location I tried to install the HS210 at. This side has a neutral, where the other doesn't, so this is where I installed. It might be a bit difficult to make out, but you should be able to see that the 3-wire run to the other switch is red, black, and white. Where red and white are my runners and black is actually hooked up to the neutral from the 2-wire run from the light fixture. Black from the 2-wire run is hooked up to common. 
 

Here is the other end where only the 3-wire run is coming in, no neutral. Runners are still red and white, black is my common from the neutral on the other end. 
 

This configuration with the dumb switches works as expected for a 3 way switch setup. However if I hook up the HS210 on the side with the neutral and have everything hooked up the same, and add the neutral wire into the wire nut with the others, it will flick on and off repeatedly similar to this video. 
 

/r/TPLinkKasa/comments/s5js69/hs210_keeps_clicking/

(Can't post actual link)
 

I was not able to find a resolution in this thread unfortunately and the few other threads I'd found mentioning the same issue didn't really seem to offer any resolution either. 
 

To help describe my wiring a bit better, here's an image that is almost identical, with the exception of the with and black wires in the 3-wire cable are swapped. 

Please let me know if I can help provide any further info.