The "No Stone Left Unturned" achievement in Tomb Raider: Definitive Edition.

The issue is here:

I have acquired all documents, relics, and GPS caches, as required. However, one relic in the "Shantytown" section of the game is still marked as uncollected on the map. When I try to collect it, nothing happens.

In addition, a tally on the map indicates I've collected 7 of 7 relics, and 14 of 15 GPS caches, even though all GPS caches are marked as collected - and the one marked as not collected - is a relic. Very odd.