Using the commands, I summoned an armor stand using:

summon armor_stand ~ ~1 ~ {CustomName:"\"Test1\"",CustomNameVisible:1}

and I rotated it using:

execute as @e[x_rotation=0,name=Test1] at @s run tp @s ~ ~ ~ ~90 0

Because of the target selector for the entity that the command is rotating includes "x_rotation=0", the entity should only rotate once, because it spawns with an x-rotation of 0. However, the command rotates the entity multiple times, which shows that even though the entity appears to be rotating, the x-rotation of the entity is not. This is seen visually when I logged out of the world and back in, where the direction of the hitbox of the entity was NOT aligned with the direction the entity should be facing. I also tested this with a creeper (including "NoAI:1b" in the tag of the summon command) and the same thing occurred.

I demonstrate the bug in this video: Rotation Bug Demonstration (Youtube)