Why does the roll and yaw/heading values show large errors when the pitch approaches ±90 degrees?

This is caused by an (mathematical) artifact in the definition of Euler-angles (also known as Cardan angles). In the Euler-angles description of orientation there is a singularity (also called "gimbal-lock") when the pitch is +90 deg or -90 deg. This means that, as the pitch approaches +/- 90 deg the output angles in the yaw and roll will start to fluctuate greatly and that a pitch of +/-90deg will not be reached. Euler-angles are mainly suitable for intuitive visual interpretation.

The orientation output modes Rotation Matrix or Quaternions are required for further calculations and processing and do not suffer from this purely mathematical artifact.