That is a little over half charge. Since you are implying you can still ride the bike with "reasonable" performance, I wonder if your voltmeter is just wrong (low internal battery). Does your bike have a battery status display and is that correspondingly low, like half its usual value? What would the charger voltage be?
If it's real, then someone has to measure all 13 groups to start figuring out what's wrong. Be careful.
You might try a long balance charge first. You got 13 groups of cells in a 48V battery. The BMS shuts off charging when any group reach the max voltage, 4.2V. In a balanced battery, they all get there about the same time. In an older battery, one gets there first. The others are a bit slower, but if you leave it on charge, the balance circuits bleed down the first one and let the others catch up. When a battery gets really unbalanced, it make take a day or two or more on the charger to balance them.
I'd do this somewhere where it's safe, like outside. Battery charging is generally safe, but if the battery and charger both go bad, it's not good.