Appointments + mulitiple service providers causing issues

Hello beautiful people!

Hopefully someone can help me here as I'm getting the urge to throw my Macbook!

I'm trying to set up a booking system using Appointments +. What I need to do is be able to take multiple bookings for the same time slot. But for some reason as soon as I add another provider (dummy or otherwise) to that service, it messes up the booking times and suddenly half the slots aren't available anymore.