EDIT: Oh shoot, this is a duplicate of MC-3416. I combed through ~250 map bug reports to make sure this wasn't a duplicate, and somehow didn't see it. How embarrassing. ;- _ -

As several map markers are added (by hanging map copies), some of the markers get lost. I've seen it happen with as few as three markers.

Marker icons can still be partially seen from some angles, but from most angles they are invisible. In the attached screenshots the marker in the four o'clock position is bugging out. It appears the same effect happens on maps in hand, but they just disappear.

It might be some kind of z-fighting given that it is extremely sensitive to position and angle of view.