The bug
When you throw an ender pearl while you mount a mob, you are dismounted from the mob and are not teleported by the ender pearl.
How to reproduce
- Mount any mountable mob
- Throw an ender pearl
Example
Here is an example in video: Minecraft 20w20a - Singleplayer 2020-05-13 17-02-18.mp4
Note: It works correctly in 1.15.2