The bug

Mobs spin on themselves endlessly when they are on sea grass.

How to reproduce

Just spawn a mob on a sea grass or a tall sea grass.

Examples

Here are some examples in videos (not exhaustive):

• Wandering Trader: Mobs spin on sea grass (1).mp4 Mobs spin on sea grass (2).mp4
• Natural Wandering Trader: Mobs spin on sea grass (4).mp4
• Villager: Mobs spin on sea grass (3).mp4
• Cow: Mobs spin on sea grass (5).mp4
• Pig: Mobs spin on sea grass (6).mp4