Lightroom: Finding errant synced images/mismatched sync counters

I've switched back to using my old Classic catalogue again, and ditched Lightroom CC for now.

I completely deleted all the uploaded originals I had on there (around 55,000, frustratingly enough), and started afresh with syncing Smart Previews from Classic.

I'm up to about 38,000 at the moment, but starting to get problems now, mainly I cannot keep the sync counts matched between Classic and CC. I did have a mismatch of 3, and managed to find the errors, clear them and resync (by removing from the collections, and then putting them back again). I had tried Reset Sync Data in Classic, but that didn't work.

That sorted it for a few days, but now it's back again, and increasing in value. Again I got 3 extra images in CC compared to Classic, but no idea why, there are no errors showing anywhere. Today I found three images in the 'Sync Errors' collection, which I sorted by clearing from the synced collections, and then putting back in again (from the Folders section). But that's now added another 3 extra to the CC count.

I currently have 38742 images in Classic's 'All Synced....' collection, but 38748 in the CC 'All Photographs album' (this is consistent across CC devices too, in Lightroom CC on my Desktop, iOS and in Lightroom CC web).

There are no syncing errors showing anywhere, so I have no idea which images are causing the count error - none of the filters in Lightroom CC are helping either, as all images, according to that, are synced correctly from Lightroom Classic.

Any ideas?