The bug

When the player has exactly a y-rotation (yaw) of -90 (= facing east) they do not take damage when colliding with a wall while flying with an elytra.

This can be seen in "Minecraft 1.16 Sixteen Nether Update Myths [Minecraft Myth Busting 128]".

Reproduction steps

1. Fly in an area with a wall in the east
2. While flying set your y rotation to exactly -90 (don't move your mouse afterwards)

   /teleport @s ~ ~ ~ -90 ~
   

   You do not take any damage when colliding with the wall

Reproduction steps (complete setup)

These reproduction steps describe a complete setup, but might not work reliably if there is latency or if movement mechanics change in future versions. Try the "Reproduction steps" above in case the following don't work.

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

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

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