Passkey issue when logging into to a site with multiple accounts.

Greetings,

I have two Nintendo Accounts saved to iCloud Keychain, both with passkeys (their support page suggests I reach out to Apple first). One account is the US site and the other is the Japanese site, the only differences between the two accounts being one is an @iCloud domain and the other is an @outlook domain, as well as the passwords being different. Both accounts correspond to the .com and .co.jp versions of the site respectively, but it seems both sites use the same accounts.nintendo.com page for authentication. For some reason, both accounts have been erroring out when trying to login via passkeys, though traditional 2FA works fine. All other passkeys for my various accounts work fine, so I was wondering if anyone had insight as to what might be causing the problem.