Okay guys, I went deeper down the rabbit hole and it gets more interesting.
So the relics in solo campaign used to spawn randomly (5 of them for 6+ spawn points per map). Now, they have fixed spawn points (5 for 5).
H O W E V E R
The current system kept a little souvenir from its predecessor: the relics aren't specifically tied to one map, and if not found, they will spawn on the next chapter.
This means you might not find the same relics as in these online guides if you missed one on the way.
This isn't relevant to our brother @Stinja since the missing relic is in chap 9, right?
Oh wait, the tome is supposed to appear from chapter 2 onwards
So it might be missing because the game considers you already have it, and it moved the relic queue by one item, meaning there is one less relic than spawn point in total.
Just a theory (I repeat: just a theory) but hey, we're going somewhere. We will get to the bottom of this.
Cheers for looking into this, but I've run through the SP campaign again, got one glitched "missing" relic on chapter 8 (the south-most one, in the big room), but a restart chapter made it reappear.
No bugs on Chapter 9, well only the bitey sort , got all relics, got the Time Lord Trophy, and then platinum popped straight afterwards - yay!
Now I can just chill and do multiplayer (although I do need to hunt arch-tempered Kirin in MHW).