The bug

Relative values can be passed into the y-rotation and x-rotation arguments in /summon, but only when /execute is used. Otherwise, the relative values are assumed to be 0.

Steps to reproduce:

• Run the following command from chat. This should inherit the player's rotation for the y- and x-rotation arguments:

  /summon minecraft:pig ~ ~ ~2 ~ ~
  

→ Pig is spawned facing south, y- and x-rotations are 0.

• Now add /execute run to the beginning of the command. Note that this did not change any contextual components of the command.

  /execute run summon minecraft:pig ~ ~ ~2 ~ ~
  

→ Pig is spawned in the direction you were facing.