Vendor output is not stacking

[Polyana-Fontes]

Steps to produce:

1. Setup a vendor block to sell 1 item for 1 coin and add at least 64 items to the stock
2. Change the block owner or use an other account
3. Add enough coins to purchase a full stack
4. Click on the buy button normally (without shift+click) multiple times. The operation will only work when you remove the item from the output slot