When I try changing my primary email, when I attempt to authenticate with my yubikey, I am sent to a blank page, whereas if I try with totp it works.
Trying to authenticate with a yubikey sends me to https://example.com/u/confirm-new-email/ instead of the yubikey authentication page (or it is the correct url and it’s not displaying?)
Same thing happened to me. Trying to use webauthn fails with a POST returning 400. Falling back to TOTP works, so it’s only reproducible on webauthn path.
What browser and security key are you using? I just tested this here, with YubiKey 5 NFC and Chrome on Linux and MacBook Pro’s fingerprint reader and Chrome and it worked.