Single milk bucket can't be put in item frame.

If you milk a cow with a single bucket and then try to put that bucket of milk into an item frame, an empty bucket then appears in the frame. The milk disappears, even if you pick up the bucket. However if you milk a cow using two buckets, the first bucket of milk you place in the item frame does the same thing, the milk disappears but the second bucket of milk in your inventory can be placed into the frame full of milk with no issue.