How to reproduce:

1. Execute

/tp @s ~ ~ ~ 0 90

to put your viewpoint at the correct rotation.
2. Run

/say @s[y_rotation=0]

and

/say @s[x_rotation=0]

and observe that they both succeed, even though one of the values is set to 90.

One should appear as being false, as it is set to 90, not 0.