I wonder if the battery could drop a cell but still show as "charged" on the trickle charger i.e. all the good cells are charged but the bad one(s) are just ignored by the charger
Cells are connected serially to get to the 12v nominal voltage, if one dies there is no electrical continuity any more and the battery dies too. There is no way the charger can access a single cell.
Ciao,
Dg800