HUB_DISCONNECTED from Keen Smart Vent

One of my Keen Smart Vents, connected to ST, is reporting Status - HUB_DISCONNECTED but it seems to function properly. Any idea what happened and how do I get it back online?

I am using custom device handler My Keen Home Smart Vent.

Sometimes that message take a little while to go away. Prior to the latest (and buggy) hub firmware release it would take a hour (or so), but now it can take hours (or days).

That happens when your hub has rebooted and/or been off for a few minutes.