I think they completely broke "Do not disturb" in iOS 15.
Until iOS 14.8 I've always had Bedtime between 00:00 and 7:30 in the morning and DND between 22:00 and 9:00 in the morning. They always worked perfectly.
After upgrading to iOS 15 first of all I had to re-create my DND configuration: I set it between 22:00 and 9:00.
But... it didn't work at all! This morning, after waking up, I realised that around 8:00 in the morning, DND was already OFF.... why?! Well... I put it ON manually and I expect it to go OFF again at 9:00, but..... it doesn't work either!
Essentially I created a schedule for DND but it's being completely ignored.
See the screenshots:
In the left one you can see it's 8:12 and DND is already OFF. In the right one you can see it's 9:01 and it's still ON.
View attachment 1839991
View attachment 1839992