on behalf of profile: adv multi-select fails with preset options

to replicate:

• create a custom data field of type advanced multi-select
• add the field to an organization profile and mark it required
• create a contrib page with on-behalf-of enabled and use the same profile
• with your logged in contact, add an org as employer and permission the indiv to modify the org
• create some data for the org in this field

now visit the contrib page. the advanced multi-select field for the org will appear prepopulated. but if you try to save the form, it will be returned indicating that field is required.

the existing values are not getting pre-set properly, and thus are lost on form submission (or validation fails).