I played multiplayer via LAN and was exploring when the host's Minecraft stopped working (turning white and not responding). He closed the program and opened it and the world again. After joining I looked at my map and the areas I had already explored didn't show up anymore, also the marker that should show my position was gone.
I thought I was outside of the map's range so I made it larger. As I looked at it it showed terrain again and as I walked around it generated as normal but there still was no marker.
The weirdest thing is that other maps in the same world work completely fine and are also still completely generated (show all the discovered terrain).
Cannot reproduce