Auto fill

If your iPhone is up to date AND if you already checked all the "autofill" settings in Safari and in Passwords (as per this support page:--> Automatically fill in forms in Safari on iPhone - Apple Support ) I'd try also this workaround: sign out from your Apple ID, restart your iPhone, then sign back in to your Apple ID.

Regards

Giulio

Have you already tried a "forced" restart of your 8+, using these instructions?:--> Force restart iPhone – Apple Support

Regards

Giulio

Yes, by signing out you only go out from your Apple ID (from Settings>[your name]... last option at the bottom) and you are not modifying anything on your iPhone NOR on your Apple ID.