I've always considered it a bug in AC21.
Daniel, here is how to replicate it:
Take the detail tool and draw a rectangle, clicking somewhere to place the marker (or anchor point).
Select it and change the marker rotation from horizontal to rotated, making sure relative to orientation and 0° as angle are both set.
Now change the view orientation angle and watch the marker go bonkers.
The good thing is it is fixed in AC22.
The bad thing is some of us are still finishing projects started in 21.
ARCHICAD 27 INT
macOS Sonoma