This is a rare issue when an NFC session remains open in the background, that can be fixed with a hard reboot. On most modern iPhones (8, X, 11, 12, 13, 14, 15, etc.), you force‑restart like this:

1. Quickly press and release the Volume Up button.
2. Quickly press and release the Volume Down button.
3. Press and hold the Side (power) button.
4. Keep holding until you see the Apple logo, then release.

After the reset, your NFC scanner should work again.