Files marked as missing when they're not, and then Lightroom can't reimport

Sync folder shows: 4x files missing. I chose show missing files. However, the files are not missing, they're in the catalog and the usual 'missing' icon isn't there. If I choose 'show in explorer' LR opens the file in explorer. The file is clearly not missing!! The file locations in LR Classic and Windows Explorer are aligned.

I optimised the catalog and the behaviour was still there. And it gets weirder.

I synchornised the folder and this time chose to remove the 4x 'missing' files. All 4x files were removed from LR. I then synced the folder again and chose to import missing files. Only 2x files are now showing. I can see the other damn files in Windows Explorer, but LR won't let me import!

There are identically named DNG files in the same folder (but surely LR should be able to tell a DNG and JPG apart?!)

Any ideas? The attached video shows some of this on my missing (according to LR, but not really) self portrait.

Other observations: This is has also happened in a different folder and across files from several cameras  / smartphones.