The bug

When you are exactly having a y-rotation (yaw) of -90 (exactly the positive X direction) you don't take kinetic damage when gliding with an elytra against a walI.
This can be seen in Xisuma's video "Minecraft 1.16 Sixteen Nether Update Myths [Minecraft Myth Busting 128]" at 11:52.

Reproduction steps

1. Change your gamemode to Survival mode
2. Glide with an elytra towards a wall in the east
3. Run this command before you hit the wall: /tp @s ~ ~ ~ -90 ~
4. Make sure to not move your mouse
   You won’t take any damage, but when you move your mouse to change the direction, you will take damage.

Reproduction steps (complete setup)

1. Change your gamemode to Survival mode

2. /setblock ~2 ~ ~ repeating_command_block{auto:1b,Command:"teleport @p[nbt={FallFlying:1b},distance=..2] ~ ~10 ~ -90.0 20"}
   

3. /fill ~18 ~ ~ ~18 ~3 ~ sandstone
   

4. Equip an elytra
5. Stand on the command block and start flying (don't move your mouse)
   You do not take any damage when colliding with the wall