Adding markers to maps via banners works well.
After I destroy a banner that had a marker on a map, the marker usually disappears from the map. After a while (I assume reloading the game or leaving the chunk), the marker will be on the map again, although the banner does not exist anymore.
If I get into the chunk of the former banner again or come close to its former location, the map icon disappears again. But it will reappear later on.
Comments 3
This is an issue again in 1.14+, destroyed banners that have been marked on locked maps are not removed, however this effect is immediate and does not require any re-logging. I don't think this is an intended feature of maps because of this comment I found on MC-141989 where Josh Letellier mentions "The map itself cannot be changed once locked, however banner markers are intended to be allowed to be added or removed."
Is this still an issue for you in the latest snapshot or release?