Logging into Nextcloud through Safari and Chrome with a device over NFC works fine. However, the built-in browser for the Nextcloud App shows “Browser not supported” / “Passwordless authentication is not supported in your browser”.
I found a few topics on the bug tracker regarding the 2FA and the new login flow that was introduced some time ago (if you use TOTP for example). That was all solved. However, there was nothing with NFC. Not sure if you want to use this permanently (e.g. each time you use the app, or on some other regular basis), or just that it can set up an app password.
The best is probably to create a feature request on the bug tracker: