Summary:
Starting from version 1.21.2, the x-rotation in the /teleport command, when rotated using relative angles, is clamped to within ±90 degrees of the entity's original x-rotation.

This bug may cause existing datapacks to malfunction.

The red sector in the attached image represents the range of possible rotation.
In version 1.21.1 and earlier, the range of rotation was from directly above to directly below.
However, starting from version 1.21.2, if the initial angle is -70°, the x-rotation can only be lowered to 20°.

Steps to reproduce1:
Execute the following two commands in the chat box.

/tp @s ~ ~ ~ ~ -70
/tp @s ~ ~ ~ ~ ~120

Expected results1:
The player's x-rotation will be set to -70+120 = 50.

Observed results1:
The player's x-rotation will be set to 20.

Steps to Reproduce2:

1. Place an armor stand in the world.
2. Make the player's orientation face directly downward, and execute the command: teleport @n[type=armor_stand] ~ ~ ~ ~ ~. This will set the armor stand to the same position and orientation as the player.
3. Next, make the player's orientation face directly upward and execute the same command again: teleport @n[type=armor_stand] ~ ~ ~ ~ ~.

Expected Results2:
The armor stand should face directly upward, matching the player's orientation.

Observed Results2:
Instead of facing the same direction as the player, the armor stand ends up oriented horizontally.
 

Note: This issue does not affect the /rotate command.