Okay, so.. I had the same issue. And I feel stupid and like I wasted Apple's time-- the lady was really nice and helpful. Here's a key step for anyone reading:
RESTART YOUR ROKU AFTER REINSTALLING THE APP. I'm not sure why-- she tried to explain it to me, but the gist is this: anytime you're still experiencing the same bug, restart the unit itself, WITH the software installed. Because chances are, it'll regulate or sweep out any malfunctions that were reoccurring with the reinstall.
So what worked for me was, uninstall, restart Roku, reinstall app, RESTART ROKU AGAIN (with app on there). To be safe, yes, let them send their analytics.
After that, it instantly generated the mobile sign-in code/QR code and the rest is history. Success! Hope this helps someone.