Auto Complete Field doesn't exist in FormBuilder
If you have a custom field that is an autocomplete multiple values, formbuilder won't address that. It appears that single selects are the only option for forms.
Here is an example: I have contacts, and added information is additional casts traveled in. In the backend contacts edit, no problem, you can add multiple casts per contact.
But in a form you only can use a select, not multiselect and it doesn't inherit from the base.
Therefore, members can't update their casts with multiple casts.