I recently hit 100 on a new seasonal character. Multiple glpyhs leveled up, a few pit runs. This is just to say there should be enough time and various activities to all manuals to drop.
Usual activities that drop them do not seem to drop this last one.
I am beginning to suspect there may be a bug with the game detecting the manual is already in codex and doesn’t drop the last one, maybe some weird interaction with an eternal character that has already dropped this manual.