Items are not stacking in groups of 64 in Inventory, chest or other item holding entities.

when an item is placed in a chest, and that item is added to the inventory,   the item is placed in the first availible space, regardless of where the item exists in the inventory, and does not stack in pairs of 64, but odd numbers like 17 or something else.

to reproduce:

1)place a row of items (any item, no item is not impacted), for example, cobblestone in a chest on the top row, leaving one or several spaces empty on the first row

2)place any other item in the second row,  for example, diorite. 

3)from the player inventory, with the chest open, quickmove a stack of the second item (diorite) into the chest

the diorite item will be quick moved to the first availible space on the top row, instead of being paired with the diorite stack already existing in the chest.

additionally,   when crafting items, like sticks,   the last item crafted will be moved into a separate stack

to reproduce

+

1)grab wood planks,   quick craft into sticks

2)the last pair of sticks crafted will be moved into a new inventory spot, even if the number of sticks in the first inventory spot do not exceed 64.