It is not possible for a scheduler to see which services a volunteer has chosen to serve at from the Group Scheduler. This can lead to schedulers assigning people to incorrect services where the person is not available.
I can see two options to helps solve this:
When "Show Matching Preferences" is selected on the Group Scheduler, Rock only respects the group member's weekly schedule preference when taking into account whether to include a person on the people list, it does not filter out people who prefer not serve at the selected time.
It would be helpful to have an additional option of "Show Matching Preference using Week AND Time" or in some other way designate whether the person's time preference should be taken into account.
In addition to, or instead of, Option 1 it would be helpful to add an indicator next to a person's name if they have indicated that they prefer to serve at the selected time.
In addition to these updates to the Group Scheduler it would be helpful to add the time and location preference to the Group Member Detail Block so that schedulers can view and edit these preferences without having to go to the bottom of the Group tab on a person's profile.