Relevant details hier… I am using OH 3.2 and Zigbee via conbeeII/deconz (on the same virtual machine).
First, what I really want/need: wife accepted standard wall switch for (zigbee, dimmable) lamp.
The existing/old switch wiring is cutting the power to the lamp, and of course OH, Alexa, etc would not work.
I want some wall (zigbee or else) switches which would fit the existing frames (Busch-Jaeger - e.g. Frame Reflex SI Linear, some of them are multiple); everything I found (e.g. Aqara Wall Switch…) would not fit, but suggestions are welcome.
Therefore I was looking for in-wall solutions to hook up to existing switches…
For standard lamps I have some smart light switches (e.g. Tuya ZigBee 3.0 Smart Light Switch, or Xiaomi Aqara Relay Module) which can be hooked up behind existing switches. The nice thing is, they always toggle by the wall switch no matter what the relay state OR the wall state is (e.g. they toggle the light/relay to ON even if the wall switch toggles to OFF).
As a workaround, I tried to use one of these without electrical load as a virtual “input” for the target lamp (which is now always connected to power).
So I have 2 things + switch items (lamp_switch and wall_switch) with their default links.
Then I added “cross” “follow” links from each channel to the “other” switch item.
So far it mostly works as intended; controlling the lamp from OH/Alexa also switches the relay, and manual change from the wall switch correctly toggles the relay again (switch->relay->lamp).
The problem is… sometimes when the lamp (and relay of course) is ON it unexpectedly switches OFF, without apparent reason!
I have the feeling that OH/deconz binding sometimes gets a wrong (timeout,error?) state from the relay, and it triggers the chain: light off → relay off.
Is there a way to search for the reason? Or to avoid such spurious changes??
I’m no OH expert, but I have technical knowledge…
Thanks and sorry for the noise 
Gabriel