Steps to reproduce:
Go into creative mode and get a milk bucket.
Drink the milk bucket a couple of times.
Observed result:
You get multiple buckets.
Expected result:
You do not get any buckets.
This behavior is inconsistent with placing down water, lava and powder snow buckets, drinking honey and water bottles and potions and eating beetroot, mushroom, rabbit, and suspicious stew.
This is intentional. However, it is supposed to check if you already have a milk bucket in your inventory. So it should only give one bucket.