Gel battery overcharge
Yes, a plausible explanation is that this buck converter or shunt regulator is damaged and through conducting. It can measure the overcharge but contoling the MOSFET switch does nothing, because it is probably conducting all the time.

It is normal that after diconnecting the charger the voltage drops, since this is the cell voltage. Even if you charge it with 20V, the battery cells can't be charged to that voltage, it'll just damage them.

You can measure at bright sunlight the panel voltage and battery voltage, and if both votages are equal, than it means that the charger is damaged.