No preferences for selecting providers

We have 3 services and 3 providers, all 3 providers are selected to provide all 3 services.

When we go to make an appointment, the first choice for providers is "No preference" with a drop down to the other 3 providers.
Love that idea, if the client doesn't care who is providing the service.

Now, if all 3 providers are booked on the same day and same time (the calendar shows them as all busy), why does "No preference" still shows that time as available for booking?

And I'm still able to make appointments with "No Preference" for that same day and time for the same service.