May i know did you created a new catalog on the desktop and synced it with Lightroom mobile.
I had the same issue today. Worse than that, after I deleted all mobile collections and created new ones, Lightroom again behaves strangely - one collection is gone (on PC) but is remaining on iPad! The sync status says "All up to date"))
Yes, I created new catalogs on the PC first and then synced them to LR on iPad.
Also, suddenly two of my older collections appeared back again (after deleting everything as requested by LR!)