What i did / Steps to reproduce

I have multiple shape markers were their borders do overlap on many occasions. (I can provide the exact marker sets if needed.)
Make the markers visible, and then turn the map around.

Expected result

Visible border lines should stay visible and invisible border lines should stay invisible.

Actual result

There are occasions where the border lines flicker heavily, but sometimes only a bit.

Context

BlueMap Version: 5.10-fabric
Browser: Firefox and Chrome

This is either z-fighting or more likely just simply the draw-order sorting not knowing which marker is in front and which one is in the back.
The way to solve this is on your (or the addon that is creating the markers) end, for draw-order sorting make sure your markers positions are not at the same y level, just give the markers you want to be on top be at a slightly higher y.
If its actually z-fighting you do the same thing with the markers shape-y property to solve it :)