I switched this off by choosing the wrong answer to some vague prompt and could never figure out how to re-enable it. Assuming it's like the many iOS settings that can be reverted only by resetting the phone to factory defaults.
It doesn’t work on all sites and apps, which is an annoyance. Why it can’t intelligently offer the SMS OTP when a user is just waiting on an input field and an SMS comes with a code is beyond me. They should be able to decipher the messages, regardless of variations in formats, and know the code.
BTW, the setting to enable or disable this seems to be under Settings->Passwords->Password Options->AutoFill Passwords and Passkeys. Turning it off and on may also work (as these things tend to behave across devices and operating systems).
