When attempting to combine items, similar items we'll swap positions instead of coalescing into groups of 64 units per block. Example: if I have 10 blocks in inventory, and 20 blocks in a furnace of the same type, selecting the ten blocks and trying to place them on the 20 blocks will swap their positions.

If you have items in your inventory and mine additional items of the same type, they will increase the block count up to 64 blocks as expected. But you cannot combine items of the same type that are in your inventory.