The thing people in this thread need to understand about software testing and patch releases is that feature development of iOS 13.0 stopped months ago. Internal testing started and more recently, external testing started.
During that time, the developers didn’t sit and wait for bugs. They started on the next part of the backlog under iOS 13.1. It may surprise you but they likely are already coding iOS 13.2. They do this while they fix bugs for iOS 13.0 and iOS 13.1.
On many software projects, changes made to upstream branches (i.e. fixed made to iOS 13.0) merge automatically into downstream branches (ios13.x). However on an operating system this likely only happens periodically due to the complexity and difficulty in merging the two code bases.
It’s possible that iOS 13.1 has bugs in it which have already been fixed in the latest beta of iOS 13.0 and won’t be merged into iOS 13.1 until 13.0 hits GM.
I get that it’s unprecedented for a patch release to hit beta for Apple before the .0 release goes GA but it’s really not that uncommon in software dev.
Finally, the commenter who said ios 13.1 will be taking the release date of ios 13.0.... that doesn’t speak ill of iOS 13.0, rather it means iOS 13.1 got done ahead of schedule. If true it means that the internal checks of the 13.1 features are complete and they feel confident that they can deliver them by the 13.0 release date. Personally I think that’s highly unlikely. But if true it’s not a bad thing.