When I right-click items from my bags to consolidate them with existing items in my bank reagent bag it indicates that the bank is full (fair, it is, but it should stack) if the other bags are full as well. If other bags have space available the item will go there instead of stacking.

My guess - the code is checking to see if the reagent bag is full before it checks if the item already exists for stacking purposes, probably just requires a logic re-ordering.