Tp with relative x rotation above 90 affects y rotation

When using tp to modify x rotation, any value larger than 90 (or smaller than -90) will affect the y rotation.

Example of expected behaviour:

• Set default position: /tp ~ ~ ~ 0 -90
• Reduce facing direction: /tp ~ ~ ~ ~ ~90

Viewing x angle is now at 0

Example of erroneous behaviour:

• Set default position: /tp ~ ~ ~ 0 -90
• Reduce facing direction: /tp ~ ~ ~ ~ ~91

Viewing x angle doesn't change, and y angle changes to -180.
I expected the x angle to increase by 91, going from -90 to 1.