It would be nice is SmartThings development explained what criteria is used to determine if a device is offline.
It is possible that device health is buggy.
Or it is possible that device health is finding an underlying marginal comms issue
Right now I am going by the assumption that it is correctly identifying something, like maybe communication to the one device that I have this is being marked as unavailable has some kind of marginal communication problems.
In my case toggling the device on and off in the iPhone app temporarily “repairs” it.
Suppose device health was actually detecting some kind of underlying communication issue with the device(s)?
So in the case you mention, it sounds like this “bad” device was the only route to one or more other devices.
But if comms to that device is in fact marginal than using it as a repeater is problematic.
It seems that since this feature was released less of my devices are being marked as unavailable, in the last few days only 1 specific device gets marked unavailable.
This could be explained if they have tweaked device health by relaxing tolerances for dropped messages or something similar.