I recently got a new PC (Windows 10). Successfully transferred my Lightroom Classic CC data over. Recently started noticing that my "Pictures" folder (not the "Lightroom" subfolder) keeps populating with phantom folders ("downloaded-smart-previews-#"). Seeing no content in them, I have tried deleting the folders manually in Windows but they keep coming back. I am thinking it is a syncing issue but not sure what to do. They are numerous so somewhat annoying when I am looking for something in the Pictures folder. I do make edits on the Lightroom mobile app from time to time, and have synced collections, which I'm hoping can be preserved.
Those folders are probably because you changed Lightroom over to a new PC, and so your synched catalog has changed as well. Lightroom Classic normally downloads all the contents of the cloud into a newly synched catalog, but it probably understands that the smart previews in the cloud are actually the same as the originals it already has in the catalog, so in the end nothing gets downloaded (but the download folders are already created).
The only solution is probably to stop synching, delete all the smart previews in the cloud (via Lightroom CC or the web interface), and then start a fresh sync.
How do I delete the Smart Previews in the cloud only?