ok if u havent read this so far its not impossible cause its happening right now so sorry. an no i didnt have any treasure maps cuz i didnt need em. i took everything off that i could so only codex would show still nothing.
so i went an bought all the treasure maps just to see if that would unbug it wanna know what i found ...........
its still not on the map AT ALL. so i guess i just did the impossible high five for proveing the impossible wrong.
Maybe it is just a corrupted file that doesn't make the map to show the last codex page. So you should find it yourself.
Try some Online map and check them.