Lightroom Classic losing files

That occurred to me too. Often this is because a destination within the "Pictures" folder has been chosen for imports (which is the, longstanding, default for Lightroom), and also because Windows' question "backup your files to cloud?" has been answered Yes.

OneDrive fthen moves everything from "c:\users\[user]\Pictures" (etc) into c:\users\[user]\onedrive\pictures" (etc).

Also, these files are no longer unconditionally present in the computer locally - since that now depends on the moment-to-moment storage management whims of OneDrive, behind the scenes.

Any of these files will report for duty if explicitly opened (e.g. into Photoshop) - getting re-downloaded from cloud as necessary. But the LrC Catalog expects all images to be "on active standby" constantly, and without needing to actively open every one. So OneDrive hears no such request, thinks it's OK to dynamically regain that local disk space, and then the Catalog sees these are physically absent from local disk. 

My suggestion is either: change the OneDrive settings for these special folders (such as Pictures), to make sure a local copy of everything will be permanently there - or, better IMO, to avoid using e.g. "Pictures" for files that are imported. And in any case to avoid using "Pictures" for storing the Lightroom Classic Catalog itself.

Then the whole complication of OneDrive goes away so far as LrC is concerned. You can still of course export images into Pictures where they will be included in your cloud backups by OneDrive.

Note: backing-up the LrC imported images (and the Catalog) will then be up to you to make happen by other means.