I'm running 10.13.5.
Spotlight wasn't finding stuff, like recent email and so on, so I rebuilt the Spotlight index for the boot volume (APFS) via mdutil. mdworker and such churned for a while, but not really that long before it fell back to say .2 cpu time (500gb) and it's still borked. Can't find applications, recent folders, etc. Oddly, it seems most of the stuff it can find is old, like 2017 or earlier old. It's also borked on an external APFS SSD.
I've also tried rebuilding via adding and removing drives in Sys Prefs. Deleted Spotlight prefs. Deleted Spotlight cache. Ran First Aid on both drives; they're fine.
I was thinking it was maybe some permission thing, but there seems to be no pattern.