Even with "Form Prefill" disabled, the "multi-select" input fields fields in my form are being preset with the first option/value for each as "selected" (by default).
So, if a user submits a form with no required fields, taking no action for any field, he/she will be erroneously submitting the form as having selected the first value in each "multi-select" field!
Also, "Form Prefill" did not appear to work when it was enabled -- the correct values in the multi-select fields were not being prefilled. Instead, it was again the first option in each which was erroneously selected.