I own a G3 Powertap but the batteries last the appropriate time I guess.
However I have the very same issue you had a couple of times with Garmin HR bands.
The customer service said it was probably due to some conductive pathway forming in the chip board after some humidity from sweating penetrated into the device because they found a bit of corrosion there. Another possibility was that the battery might have leaked causing the same effect.
I prolonged the life of the last band after that happened by removing the battery after use and inserting it back just before the next workout which was a pain. In between workouts I kept the band in free air and sometimes I even used a hair dryer to make sure no moisture remained.
It is a dangerous game because the heat can be too much and the plastic can start melting.
Getting back to the hub and assuming the same effect comes from a similar cause, I would check the red o-ring seal to see if it is ok and look inside the circuit board with a magnifying glass with a help of a direct light looking for any possible corrosion.
In desperation I might apply the same recovery protocol that is used when a phone is dropped into water.
Removing the battery after the rides might help but would probably be too troublesome.
Using a hair dryer with hot air is something I would not do because I think the electronics and casing look much more sensitive than the ones in the HR band. Maybe a cold air blow helps, and is worth a try.
I do not know if it helped but at least it might give you an idea of a possible approach to the problem.