The bug

The player marker / icon on maps created in the nether is supposed to rotate all the time. However, other markers and icons are rotated randomly as well when added / updated.

It might be intended for the green item frame marker, but is is very likely not intended for markers which normally cannot be rotated, such as banner markers.

How to reproduce

1. Create a map in the nether
2. Right click a banner
   → The banner marker is rotated

(You might have to add multiple banner markers in case they randomly got the correct rotation)