That's especially weird. I've had Google decline TOTP/Google Authenticator and SMS one time when I was troubleshooting a OAuth issue, but declining U2F? Are you logging in from various different VPN servers daily, or just through the same few ISPs?

No VPNs, just my home network with an IP address that rarely changes. What seems to throw it off is when I log in from "conflicting" platforms, particularly iOS + Android. I also have multiple iPhones for work, and it very much dislikes that.

When it gets in this state, nothing will work besides going to g.co/sc on Android--it can't be any other platform, regardless of how long I've had the device--and approving the code request there. If I approve it from any other device, even with a YubiKey, it'll give me a code on g.co/sc, but I'll be told it's invalid and I'll get one of those emails telling me the code was correct but declined due to suspicious activity.

I appreciate the attention to security, but c'mon, it's a YubiKey, and I'm logging in from my usual residential location.