I seem to remember we never really found out why it got reversed in the first place.
It is a bit of a drastic situation, I suspected at the time that it may survive better left alone than risk another drastic event trying to get it back the right way. It is not ideal reversed, you have outside positive plates instead of the normal situation with a negative each side on the outside as normal. In addition the plate technology is supposed to be different but some cheap rebuilt batteries used to be built up of reclaimed negative plates for both.
At least if it keeps going for a while that is encouraging. Not sure if your quoted voltages were standing with no charge or discharge for several hours or whether it has become a 10v battery pushed to gassing, that may be what is happening.
Flux