In addition to JJuice's answer above, it is also required for the password text field to be on the same screen as a username/phone/email text field (and therefore in an AutofillGroup with it). It seems that if a password field exists on its own - even if wrapped in an AutofillGroup - iOS won't currently recognise it as a sign up form. This caused me an issue as the previous sign up flow for my app gave each text field its own screen, which is not uncommon.
I will also flag that Autofill Credential Provider was not required in my case to get this working. I believe this would only be required if you're building eg a third party password manager.