I have the same problem. I use special slots to change background of slots for weekend days, holidays and so on. I have special slots for it and have implemented ScheduleViewStyleSelector / SelectStyle and defined templates with different colors for background.
It works fine without grouping but not for grouping. Adding of Copy and CopyFrom to special slots does not help me.
I add breakpoints to program and I have found that when grouping is on then is never called SelectStyle (without grouping it is called as expected). There is called only Copy for Slots.
When should be filled collection of special slots? Before adding of appointment or after?
Is possible restore link for sample ClientsFiles/317515_slots.zip (see above) or send me another link for running sample?
Is there another way to change background color for slots?