Found when trying to make tamed parrots sit in a 'birdbath' which included a waterlogged slab surrounded by signs to keep the water from flowing. I noticed that the parrots were never in the birdbaths whenever I left and came back. After watching them more closely for a time, I spotted that they were moving on their own even though they were sitting. They even remain in the sitting animation as they move or fly away.

I tested sitting parrots on other blocks like dry slabs and walls, and found that only those in contact with water seem to 'forget' that they're sitting. Most often, I found that unloading and reloading the chunk is the most reliable way of replicating this glitch, though I have seen them start moving when I've had the chunk loaded for a while.

Steps to Reproduce:

1. Place down a slab and waterlog it (encase it in signs to stop water flow)
2. Summon a parrot, and tame it with seeds
3. Make the parrot sit down
4. Push it onto the waterlogged slab
5. Save and Quit, then rejoin the world

Expected:

The parrot would continue to sit on the waterlogged slab.

Observed:

The parrot will start flying away with it's sitting model applied, until it lands. Once it lands, it will continue to sit as expected.

Notes:

1. Related to: MC-174495 MC-267827 MC-119779 MC-206505
2. This issue can also occur when leaving the loaded chunks and coming back, but is quicker to reproduce with rejoining.