https://html.spec.whatwg.org/multipage/custom-elements.html#form-associated-custom-elements:face-state-6

When the user agent believes it is a good idea to restore a form-associated custom element's state, for example after navigation or restarting the user agent, they may enqueue a custom element callback reaction with that element, callback name "formStateRestoreCallback", and an argument list containing the state to be restored, and "restore".

If the user agent has a form-filling assist feature, then when the feature is invoked, it may enqueue a custom element callback reaction with a form-associated custom element, callback name "formStateRestoreCallback", and an argument list containing the state value determined by history of state value and some heuristics, and "autocomplete".

This bug is filed to figure out how FACE would support form restoration or autocomplete.
I think this won't block FACE shipping.

Form restoration for FACE is going to be handled in bug 1556358, so this bug is now for autocomplete support.