I can successfully scan for new zigbee2mqtt devices. They appear in the web ui, I can successfully rename them, and use the the device but after a while the new added device disappears again from the web UI. If I fiddle with the join button (the one with the dropdown next to it) and having the (Tuya) device in pairing mode again I am able make it appear again in the web-UI, it even remembered the name i gave it.
But after a while it disappears again.
Do I have to somehow mark the device that it is a permanent part of my configuration? (this might be a very simple issue I am not aware of)
Actually the device i try to permanently add it is a Tuya TS0001_switch_module
( there are no problems with the TS011F_plug_3 devices which are listed in thge screenschot )
Again, you don’t mention what specific model of Sonoff dongle you’re using. Sonoff make quite a few zigbee coordinators, so you need to help us help you.
Anyway, you should be able to get the info by searching for your coordinator in HA > Settings > Devices & Services list.
Your coordinator device page should have this listed in the Device Info section, next to the Hardware field: