I’ve seen this happen even with the v2 hub and some thermostats. Apparently it was a signal issue according to ST, the only solution I could find was to bring the device closer while pairing. According to ST there was an disconnect between the hub and the cloud due to an incomplete pairing process which was likely caused by the signal issues.
Tagging @Kianoosh_Karami and @carter.swedal who may be interested in looking at your hub logs to see what’s going on and why it isn’t showing up.