I have a formula that determines the date of the first day of the week for a given date.
ED := EventDate;
@Adjust(ED;0;0;-(@Weekday(ED)-1);0;0;0)
This works just fine for a single value field but it does not work for a multi value field.
It seems like it is using the weekday of the very first date in the list and subtracting that from each value in the list as opposed to subtracting the weekday for each of the member values.