My iPhone only puts in the first digit of the code

The code boxes are built to take the whole six-digit code at once, whether it is pasted in or filled in by the phone from the email. The code in the email is ordinary text, so it can be copied.

1. Tap the first box.
2. Paste the code, or choose the code the phone suggests above the keyboard.
3. If only part of it appears, type the digits in directly. Each box moves on to the next.
4. Choose Verify Code.