all side quests complete but the achievement is still not unlocked

[Ghostvenger92]

is there a secret side quest somewhere?

[SIRCAM]

Check this guide and verify which achievement left to complete.
https://steamcommunity.com/sharedfiles/filedetails/?id=2458847264

[spirouac]

secret quest in the canyon pillar have to unlock the 3 pillars to open the door and get an exotic chest.