Edit: solved to my satisfaction in less than half an hour!
Any #jellyfin nerds out there know why a newly-added film might be visible to one user with access to a library but not to another? Does it matter that the admin user is the one that can see it?
I cleared caches and all sorts of things. Infuriating bug, not least because I now have try and knead the pizza dough instead of sneaking some work/study in this hour or so.