I have had this issue twice. The first time was resolved when I updated my Monterey operating system. Recently it happened again. I believe I know what caused it, at least in my instance. I created a file in the FILES app on my phone and immediately deleted it. I don't think it had a chance to upload to iCloud and when I check the folders on my M1 Air, Waiting to Upload appeared. This time I shut off Desktop and Documents on both Apple devices that I had syncing (M1 Air and iPhone). I deleted the Documents folder as that was the one hung up. I reloaded the documents from a backup that I had created prior. Once everything syncs all was well again. It was a pain and I only have about 15GB of documents. Hopefully Apple will get this worked out. I'm not holding my breath as many of these posts go back years. In the meantime, I'm going to give everything plenty of time to sync before I delete files.