I fixed it for now by rolling back to the previous version of Windows 10. Yep, my mirroring issues were fine once it rolled back the update.
I now live in fear that at any moment, it will update itself again. I see zero way to stop windows from updating itself when I reset, but it ruins my ability to mirror when it does it!
Please tell me there is a way to stop this thing from updating! I just want everything to stay as it is!