Could you create it as two different types of appearance to allow the creator of the particular form to make that decision and allow the choice of to display checked boxes or not? That was they could inform their users more accurately for each case.
Or when there's a select_multiple with autocomplete appearance have it show a summary field for clarity just below the drop down list?
Because this would be a great option, I just was working with a form where I want a user to be able to select multiple asset IDs if necessary from a list (if there are multiple signs on a single support) and because I forgot autocomplete doesn't apply to select_multiple completely froze Survey123 cause it tried displaying 20,000+ records.