The bug
When using a bucket in creative mode to pick up a fluid from within the world, a maximum of one (1) buckets appear in the inventory. However, milking cows does not have a limit on the amount of buckets / bowls provided, which does not seem consistent.
If a bucket of milk / bowl of mushroom stew is already present within the creative player's inventory, any subsequent milking of a cow should either just not give a milk bucket / mushroom stew to be consistent, or milking should be denied outright (i.e. no hand animation nor sound plays).
Related issues
relates to
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This is the same with Mooshrooms and Mooshroom/Suspicious stew, should this be included in this issue or a new one?

Can confirm for 20w51a.