Hello Kbmlt,
Thank you for using Apple Support Communities. We'd like to help you with your verification code auto-fill issue. Allow us to recommend a couple of troubleshooting steps to help sort this issue out.
1.) Navigate to Settings > Passwords > Password Options and turn off AutoFill Passwords and Passkeys.
2.) Restart your iPhone: Restart your iPhone - Apple Support
3.) After the restart, turn on AutoFill Passwords and Passkeys again.
4.) If your issue continues, make sure that your iPhone is fully updated. This can usually help with unique issues, since updates include fixes. Just in case, this link can guide you through the update process: Update your iPhone or iPad - Apple Support
1.)Back up your device using iCloud or your computer.
2.)Plug your device into power and connect to the internet with Wi-Fi.
3.)Go to Settings > General, then tap Software Update.
4.) If you see more than one software update option available, choose the one that you want to install.
5.) Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do: If you forgot your iPhone passcode - Apple Support
We hope these steps help you. Thank you for using Apple Support Communities. Take care.