Zigbee radio reboots started again a few days ago, two weeks after the last reboot, just like before.
So far they are not yet repeating frequently as they did before. Memory has been holding steady at around 185-190 MB the past few days. Nothing special about CPU or other indicators I know of.
However the interesting thing is that I can currently generate a zigbee radio reboot "at will" simply by triggering a cloud backup (manual backup doesn't appear to trigger it reliably). The last two I just triggered as I was writing this reply. Only one of the reboots in this list seems unrelated to backups, at 13:54 (I happened to be using the hub's web interface via remote admin in the mobile app at that time).
Here are the (annotated) backup times for comparison:
Somehow backups are triggering radio reboots, and cloud backups trigger them more reliably. There may be other underlying conditions I suppose, like a bad device, but I have yet to find any evidence of that.
Hope that helps @bobbyD @gopher.ny