For posterity: I had this same problem recently with a QC25 from 2015. They are out of Warranty. Poking around various Bose forums it is clear that this is a manufacturing defect that has impacted many of these units.
The Bose repair website offered a new replacement for $135. I was concerned that this unit would have the same problem as the old so I called them up. I was initially offered the same deal, but after explaining that I had seen this was a known defective unit, expressing my concerns about such a short lifespan for a $300 pair of headphones, and my concerns that another QC25 would have the same problem, they:
# Assured me that this is a newly manufactured QC25 with the defect addressed. This is plausible because based on forum posts it seems they were replacing them with QC35’s for a while; if they are dealing with mass replacement it would make sense for them to do another run of repaired QC25s.
# Dropped the replacement cost to $71 (including tax) with free shipping
# Started a new warranty for the new unit (1 year, same as the original warranty)
Long story short, it is worthwhile to contact them even if you are well out of warranty. I’m happy with the resolution considering how old the unit is at this point.