Our rotators apply roll, then pitch, then yaw, so this is expected. You'll need to work out the pitch as a rotation around the desired axis and apply that. Using quaternions can help with composing rotations like that.
Add Controller Pitch Input isn't relative to the player.
The user has provided a video of this issue occurring as well:
Result: Notice that after you add Roll Input, the Pitch Input is not relative to the player
Expected: The Pitch Input would be relative to the player
Head over to the existing Questions & Answers thread and let us know what's up.