Most chargers used some form of diode/rectifier assembly to convert the AC current into DC, and this should normally prevent any current travelling in a reverse direction back into the charger while it is turned off. I wouldn't expect it to discharge through the charger, but that doesn't mean that it won't.
It's more likely that the key switch was not turned off completely, or there is a problem with the charger or the battery's
BMS.
When it has been charged up fully, keep an eye on the voltage and see how quickly it falls. The self discharge figure quoted for the GM LiMn cells is less than 0.5% in a month, which
(according to my calculations) is less than 0.01V per day.
How long has it been stood since the battery was last charged?
Is your battery stored in a dry or damp place when your bike is not being used?
Alan