Damn 
My test switches both still work after joining different devices, see https://github.com/dresden-elektronik/deconz-rest-plugin/issues/7935
Does a restart of deCONZ help in your setup?
I’ll continue testing with another setup, right now I fail to recreate the problem with my main setup and version v2.29.1-beta.
Soo, a combination of removing the switches, rebooting and re-adding them seems to have fixed my problem. However, adding them and setting them up, especially with the help of the Hue Essentials app is still far from a breeze, as they sometimes just drop from Phoscon a few minutes after adding them. If I leave them alone for a while before configuring them with Hue essentials, they seem to stay. Still seems a bit weird!
I have the same issue for quite some time. When do you think that v.2.29.x will be released (no beta)?
The Beta is out for two month now.
After losing the RWL021 switches again the other day, I threw them out and replaced them with Ikea Somrig switches. Let’s see if that helps. I’m on deconz 2.30.2 with Raspbee on a Rpi4.