Precursor Tablets only dropping from naturally occurring mechanics?

Specifically Ritual Tablets are impossible to self-sustain when trying to do Rituals in maps by using the appropriate precursor tablets. You simply don't drop ritual tablets.
But when forcing anything but ritual on your map and you encounter a random naturally spawning Ritual you get a few tablets more than otherwise.

Is this a bug or intended behaviour that tablets in maps can't drop for mechanics that have been forced by using precursor tablets?