Some of the Unique map can only be unlocked by interacting with a Stone in the atlas map close to it after killing the Final Boss a Stone appear and you need to interact with them so after x interaction the Unique Map can be Unlocked.
There is “Temporal Tethers”, “Karui Beacon”, “Draiocht Hengestones” and the final one is an loot items called “Idol of Estazunti” so the last one doesnt matter just add it on the pickit.
here is a link to those Unique map and the requirements, i think i got all the name of interacting items right just like on this website.