"no preference" not working as it should

Hi there,
I have a service with 2 service providers, we use the "no preference" option.
Service provider 1 has 3 time slots: 10:00, 10:30 and 11:00
Service provider 2 has time slot 10:00
Service provider 1 doesn't work on Nov 11 (screenshot 1), Service provider 2 does (screenshot 2)

Yet when no provider is selected (no preference) the time slots from service provider 1 can be selected (screenshot 3)

This is not good at all and already caused a bit of trouble :slight_frown:. Can anyone help?