I just had a similar situation happen. A "Map to the Treasure Hunter" pointed me to a previously generated location where there was nothing there. If the area had been ungenerated I imagine the location would have been placed correctly but it seems like it doesn't check to make sure the chosen spot isn't already generated.
Or maybe the flavor text "Hopefully he's still there and not digging a hole somewhere" should be taken more literally and they can give you bad info but I hope that's not the case.
This happened on a new world generated entirely in 1.21.5 with no mods. I made a backup of the world file before regenerating the area and have log files if those could help, though I'm not sure which specific logs would matter here.