https://www.mobileread.com/forums/showthread.php?t=82717
Lots of people have reported various issues with battery capacity and/or faulty capacity readouts on their Kobo's. The only response to this from the Kobo team that I've seen posted is try a reset (dunno if they mean pin in the back reset, or default settings reset through the menu).
I've not, however, read of anyone having your specific problem, where the battery gauge is just stuck at full. But there are plenty of cases of odd red light behavior, so I wouldn't make judgments based on that alone.
Definitely open up a support ticket with Kobo, but all they'll probably tell you is to try a reset, and if that doesn't work go exchange the unit.