Autocomplete saves pre-populated field data.

For forms not evaluated by autofill, autocomplete will cache the submitted data even it if wasn't entered by the user, but already populated in the form on load. If autofill does evaluate the form (it is above the threshold for minimum supported field), it filters out non-user entered data before any saving (either by autocomplete or autofill occurs).

The "pre-popuated data" filter is in the autofill flow, it should probably be in the general on-submit flow. Or, autofill should evaluate all forms.