Probably just long term storage, and whatever processes cause gradual discharge in that state.
I'm interpreting your middle picture as being the situation AFTER you charged it, but BEFORE the battery people did anything? And last picture is rebalanced battery.
Main load would have been BMS, and the cell voltages in your middle picture suggest a 'kind' BMS that takes its power from the whole cell pack, not just from one cell group. Otherwise you would have seen one cell group much LOWER than the rest, either the first or the last group. Instead all groups have good voltage, and one is right at maximum 4.2V near enough, which is what stopped the charge.
Probably just tiny tolerance issues over a long time.
Ths missing info is the cell group voltages before you charged it: they would distinguish between it sitting part charged, and so likely to be OK now, and sitting totally flat, and potentially now of reduced capacity.
Let us know!