Mild spoilers. Stuff you've already seen if you're reading this guide anyway.

I don't know what causes this, but this Sukhothai photo is broken for many players. I think it'll be patched soon. I saw this fix a few different places and don't know who originally found it.

Make sure you have every other note and picture in every area of the game. Of course, you will be missing one in Sukhothai.

Head to the Vatican and fast travel to the Tower of Nicholas. Go through this door:

Head inside and go to this area:

Then go through the doorway and down the stairs. You will now be in front of the Sacred Wounds puzzle, which you have already completed:

Go to take a hint picture:

The achievement SHOULD pop!

I don't think this has a 100% success rate but it seems to be working for a few people, including myself. I hope it helps. I've heard this can also work if you snap a picture of the Fountain of Confession, but that didn't work for me. Could be worth a try!