I've seen a few people have had similar problems, but mostly have been resolved by GoPro people here.
I was syncing media from the Android mobile app, and while fast, found that it would stop syncing when the phone went idle. So, I started syncing with the Quik app on the PC, which was very slow, but, kept on chugging away for days and eventually completed. I then started getting notifications on the mobile app that it was syncing media, on checking the app, the cloud media wasn't updating and it showed one specific video as being synced. I checked the web media library in a browser and sure enough, that one item was not showing up, so, I let it be. A couple of days later, I've checked on it again, the mobile app still claims to be syncing media, Quik on the PC shows nothing being synced and the mobile app has transferred ~150GB of data in the last couple of days, all while claiming that it's syncing a single 2.9GB video named GH010022.MP4. I tried uploading the same 2.9GB file in the browser manually, it transferred the entire file, then showed a message saying the media was already in the media library, however, on checking, it's not actually showing up in the web media library and the mobile app still says it's syncing.
I've cleared the mobile app data, including removing videos from the phone, which has stopped it constantly syncing and now it does show cloud media. The Quik PC app occaisionly shows "0 items uploaded to your GoPro Plus". This one specific video still doesn't show up in the media library on the web, or in the now accessible Cloud media view in the mobile app.