When consuming an oversized milk bucket stack, the remainder bucket will be lost, leaving only the remaining milk buckets in the stack. I would expect the result to be the remaining milk buckets plus an additional milk bucket. The actual result is a loss of a single bucket.
Steps to reproduce
1. Run the following command: /give @s milk_bucket[minecraft:max_stack_size=99] 99
2. Drink the obtained milk bucket stack.
3. Notice that the result is 98 milk buckets, rather than 98 milk buckets and 1 bucket.
Yes the bucket does disappear :^(