Observed Behaviour:
When inserting a Water Bucket, Lava Bucket, or Bucket of Aquatic Creature into a Decorated Pot, the item occasionally places instead of being inserted. This only occurs with buckets containing liquids, and not any other item.
The behaviour has been observed when riding minecarts, riding entities inside minecarts, and being pushed by pistons.
Expected Behaviour:
The item should be inserted, or cause the pot to shake without placing if it cannot be inserted.
Steps to Reproduce:
Open up a creative world and fill in a line of Decorated Pots 1 tile in the air, and a parallel line of Powered Rails on the ground nearby. Put any liquid bucket in your main hand, and attempt to use it on the decorated pots while riding a Minecart on the Powered Rail.
This effect can also be reproduced by setting up a simple sticky piston facing a composter, powered by a comparator pointing out from a pair of hoppers facing into each other with a single item inside. Stand inside the composter, and attempt to insert water buckets into decorated pots while the machine is running.
The behaviour has been observed with piston motion as well. The common thread seems to be that the player is being moved by some other entity. Manual movement, being pushed by water, and riding controllable animals did not cause the same issues.