A few days ago I had the same.
I am familiar with the project-cache and it’s importance.
It was however late in the evening and I hade some beers so I do not have enough info for a decent bug report.
It seemed that a decent project-cache (about 65kB) got overwritten by a later generated project-cache.lib generated by Eeschema, (Few hundred bytes in size) even when the schematic was not saved, but I’m not sure of this and don’t want to send the few developers on a wild goose hunt.
Edit: It is indeed the same schematic as I mumbled about in:
https://forum.kicad.info/t/think-i-dorked-up-a-schematic-file/16289/4
I may try to reproduce this next week or so, and maybe even file a bug report (I’ve got a launchpad account now! )