Mark to picked people, and marked nobody

Mark to is how somebody sees a record they do not own. The owner owns it, the assignee works on it, and anybody marked can read it: read it, and nothing more. It is on leads, companies, customers, deals, tasks, visits, vendors, projects, manufacturing orders, quality inspections, subscriptions, sales and purchase documents, and on any module a business has built for itself.

On all of them, it did nothing. You opened the picker, chose people, saved, and the save succeeded, because as far as the record was concerned there was nothing unusual about it. The list of people never arrived. It was dropped in transit, one step before the part of the product that files it, and the part that files it had been waiting for it since the day it was written.

The form calls the field one name and the thing that checks the form was built expecting another. Anything the checker does not recognise is quietly discarded rather than refused, which is ordinarily a protection. Here it discarded the answer and let the rest through.