Treasure Maps in Minecraft I believe should point to the treasure that is closest to the player when they look at the map. 

I encountered this issue today on my friend's realm and replicated it on my own.  Upon finding a treasure map, it always points to the same treasure, no matter how far you travel or how many maps you find.  The treasure it points to appears to be the one closest to spawn and NOT the one closest to the player when the map is opened.

The first map we found was something around x4,000,z300 and pointed back to about x72,z69.  I took it upon myself to travel to x10,000,z70 and find a new map to make sure we found a map in a completely unloaded chunk and it still pointed back to the same location near spawn.

I checked my own realm, where i know i have found multiple treasures months ago in various locations and went to about x4000,z300 and found a treasure map that pointed to the one that appeared to be closest to spawn (about x-70,z-30) and not the closest one to the player/where the map was found.

Attached are 2 images i captured from my experiment in my friend's realm.

image 1: shows my position and the map i found near that position (x10,000,z70).

image 2: shows where the map leads, which is the same place as every other map we find (x72,z69).

We are attempting to make a conduit, which requires resources that can be obtained via these chests, and this issue is impeding our progress.