SF where an instruction outlives the reason for it
I keep coming back to books where somebody follows a rule long after everyone who understood it is dead. Canticle for Leibowitz is the one everybody names, monks copying a circuit diagram as scripture. Asimov does a colder version in Foundation, where technicians run reactors as a priesthood and the ritual is the operating manual with the reasoning filed off.
Both of those are about knowledge being lost. The case I have trouble finding is the one where the procedure still works. Nobody on the page can say why, it reads as superstition from the outside, and it is the only thing keeping the lights on.
Anything come to mind? I would especially take books where you watch the reason come apart across the timeline instead of being told it already did.