I dont wont to take over your case. But I’m running also with a raspee and some lamps from Paulmann. I also loose connection after a random time. A power cycle bring them back into the network.
So my conculsion is, that it is not a problem in the air / shields a.s.o. as they work after a power cycle normally for some time.
To me it looks, like the problem occured the first time with deconz version 2.16 or 2.17. Like some heartbeat timing has been changed in this versions and Paulmann zigbee implementation does not perfectly fit anymore. As other devices continue to work.
In my case, it is possible to control offline devices by sending a command the group, they belong to. Is this also possible in your case? If yes, then we face probably the same problem an I have no soloution yet.
(My case was mentioned here: Partial lose of connections)
Hope this helps to get an idea who to resolve the problem.