I called the UA Premier Desk, they had no idea how to fix the issue, sent me to tech support. They too could not fix the issue, and suggested I do this ^ exactly. Since I was departing from YYZ, I was told to do it in ORD since YYZ would not be "real" UA employees, but rather AC employees with UA access. So, when got to ORD, I asked at the United Club, and they told me there was no way they could scan my passport and update my account - they had no access to my account at all. I got this from both the initial agent and the supervisor. They thought I could do it on a computer (not a phone). I explained to them that tech support very explicitly told me this functionality was removed quite a while ago because some people can't get their numbers right (3s and 8s look the same to them), so the *only* way to access your stored passport is via the phone app scan functionality, and that tech support had told me to speak to them. They told me that if I couldn't do it online, I would need to call support... so, UA is pushing me around in circles...

I just tried the (I think new) checkbox which roughly says "don't save your passport information (if you select this, all previous information will also be deleted)". I won't be able to find out what's in my profile now until next time I check in to an international flight (next Monday), so I'll post an update when I do that.