This bug happens on custom explorer maps using the "explorer_map" function on loot tables.
However, it could happen on vanilla structures like monuments, buried treasures and woodland mansion, since it's basically the same function powering all of them. (I think)

How to recreate it:

• Download the attached file.
  + Use /loot give @s loot test:(hit tab)
  + Use that command again alot, then view the resulting maps.
  +* Observe: *Some of them will not have it's "goal" marker.
  + The interesting part is: Though it does not have a marker, if you use this command:
  /data get entity @s SelectedItem.Tag.Decorations, and teleport to the X and Z coords, you can see the structure there (or underground if you chose an underground one).

• Conclusion: It can find the structure, but has problems setting down the marker.
• How to fix: No idea how it happened, can't suggest anything.

This is the part where you do your job. Action!