Lightroom CC/Classic sync error

About 2 months ago I changed some settings on my camera for the way it numbers files. Since then, I've had issues with Lightroom Classic pulling the wrong photos when syncing photos from CC to Classic.

For example, I'll have a folder of engagement photos that I've imported onto CC. I want to fine-tune these in Classic and then export from there. When the folder syncs from CC, however, some of the photos will be missing and be 'replaced' by other photos I have deleted out of Lightroom completely (both versions!), and even off my computer. I have no idea why the data is getting mixed up! The random photos that are showing up aren't in the cloud either - until I sync with Classic and then they come back out of nowhere. CC continues to show the correct photos that I originally imported.

If I delete the incorrect photos from Classic, the correct photos (and I'm assuming this is because the file names are the same) also get deleted, even though they are completely different photos.

I've spoken to support 3 times already and no fix.  I've tried uninstall/reinstall, resetting the catalogue.... I'm completely up to date too. Any suggestions?