Summary:
When the player flies into a nether portal or end portal while touching the ground, the player's flying speed becomes slower. The slower flying speed is not permanent, it can be fixed by flying upward or downward.

Steps to Reproduce:

1. Build a nether portal or end portal
2. Fly into the portal while touching the ground (hold the sneak key to touch the ground)
3. Release the sneak key and fly forward or backward

Observed Results:
The player's flying speed is slower. If the player flies upward or downward, the flying speed goes back to normal.

Expected Results:
The player's flying speed should remain unchanged.

Video:
Minecraft 2021-09-14 09-32-56.mp4