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.