The bug
Maps lose all their markers when being locked in a cartography table. Affects banner markers as well as green arrow markers.
The attached pictures show a comparison between a map before and after being locked.
Steps to reproduce
Create a map
Right-click a banner with it to create a marking point
Lock the map in the cartography table
-> ❌ It has no markers on it
Can confirm. Quite annoying for when you want to lock a map, but also navigate...