Test setup:

1. Load Bagnon only (no other addons, even the optional Bagnon addons like config)
2. Load a toon that does not yet have a reagent bank purchased.
3. Place at least two of a stackable item that can be placed in the reagent bank in the bank (regular bank slots for this step)
4. Shift-click them items to split the stack, and place the split stack in another bank slot.
5. Right-click either of the stacks.

Result: The item stack you right-click transfers to you bags. This is expected behavior.

6. Purchase the reagent bank on the toon.
7. Repeat steps 3-5.

Result: The stacks recombine in the bank. This is not expected behavior.

Repeating these same steps without Bagnon results in the expected behavior both with and without the reagent bank purchased.

Repeating these steps with an item that doesn't go into the reagent bank results in the expected behavior both with and without the reagent bank. It seems to be specifically tied to having the reagent bank, and trying to move a crafting reagent when another partial stack of the same item exists in the bank.

If the stack you're trying to move would exceed the maximum stack size when combined with the stack left behind, the stack left behind instead is increased to the max number of items, and the remaining items are stay in the original spot.