Hi @SaferHarbors,
Stabilize and depth hold try to maintain a horizontal orientation unless the pitch and roll targets are altered (via joystick inputs or the roll/pitch trim button functions), so just placing it at a 90 degree angle will not work to maintain that. Note the ANGLE_MAX limitations discussed in that link - it may not be possible to control to 90 degrees.
This comment (and the surrounding discussion) are likely worth a read 