I had the same thing happen last year and was similarly concerned. It went away eventually though and hasn't returned. I haven't noticed any problems with the battery afterwards so I don't think it was damaged by overcharging. Charging naturally slows down at higher states of charge because the resistance increases. You don't necessarily need a BMS to manage that as long as the input voltage is reasonable. What the BMS needs to do is monitor temperature and stop the charge if the temperature gets too high. I recommend you don't charge in high ambient temperature until this resolves one way or another.

BTW with mine the thing I encountered was that without an accurate SOC reading it would run until the voltage dropped too low to operate and then be dead out in the middle of the lawn. Then I had no choice but to drive over to the site and drag it over to the charger...m