You don't have an extra partition, but what probably happened is that during the Windows 10 upgrade, your C partition was shrunk to expand the size of the partition immediately to the right of it, i.e. Partition 4. That's the Windows Recovery partition, not to be confused with the last partition on your disk also labeled Recovery, which is probably from your PC manufacturer rather than Microsoft. Even changing the existing partition layout without creating/deleting any partitions is enough for Reflect to stop making Incrementals on an existing set. If you want to verify that you're suffering from shrinkage
, then open Reflect, go to the Restore tab, select an image from before the upgrade, and click "Browse Image". Check the "Capacity" figure for that Partition 4 and see if it's less than the current 969 MB size.
You see "Incremental" in your second screenshot because the schedule does indeed say to run an Incremental. But because Reflect can't run an Incremental for the reason stated above, it's instead running a Full. That's why the log says "Full - Incremental specified but...." Another scenario where you'd get a Full even if you specified an Incremental would be if you connected a disk that contained no backups. You wouldn't have to create a Full manually for your scheduled Incremental to work; instead, Reflect just automatically switches to a Full when conditions don't permit an Incremental.
Yes, after that unexpected Full completes, your schedule will run as normal -- at least until the next Windows 10 release if the OS partition needs to be shrunk again to accommodate an even larger Recovery partition. And since Microsoft has said they plan to release new Windows 10 versions every March and September, this is likely to happen more frequently.
Your old backups are still there and Reflect can still restore them; it just can't append new backups to them. If you wanted to roll back, the process would be exactly the same, although from experience, I can tell you that if you want to roll back to a backup that was created from a previous release of Windows 10, I would recommend restoring your EFI (System) partition, the MSR partition, the OS partition, and the Windows Recovery partition -- or the first 4 partitions on the disk in your case. Note that if you ever did that, the backups of your system after the restore would start appending to your previous image set, NOT the latest set that reflected the post-upgrade disk layout. If you didn't have that set anymore, then the first post-restore backup would generate a new Full since again, the disk layout would be different from what's contained in the most recent backups you performed prior to rolling back to a pre-upgrade backup.