One user (Spoonpl0x on Reddit) who had this issue mentioned this:
For me, the mission "Ressurection" had both codex entries marked as completed but after a manual check, I was missing the Commander Valen codex. Loaded in, grabbed it and achievment popped. Anyone having issues with this achievment, I recommend manually looking at a list of codex entries (I used Game8) and check to see if you are missing any.
If you haven't already, give that a shot and if you notice you're missing one, go into the level it appears in and try picking it up. Not sure if that'll work for you but it may be worth trying! 🙂













