Summary
Plank arch is overlapped by a low-res version of itself, which also alsways snaps in about 15° angles with the perspective of the viewer.
See Screenshot.
This is persistant since I startet playing, so loading again does not remedy this.
Moving back and forth between islands doesn’t help.
Removing the door doesn’t help.
Removing the arch itself doesn’t help UNLESS you then save and reload the game
Settings
This happens in any graphics-preset, it just gets even uglier if you set it to ‘low’