I figured out the issue: I was using the same series ID's between the two, even though there were separate. If I make sure they are unique it works.

I'm the same boat. I have a new Mac Studio Ultra but my iOS app's main dependency currently only builds for x86 for simulators. I can't seem to force all the SPM packages it relies on to build x86 simulator versions :( Apple really needs an Xcode setting to tell SPM how to build. It seems CocoaPods can do this https://blog.sudeium.com/2021/06/18/build-for-x86-simulator-on-apple-silicon-macs/

OK, some digging on StackOverflow shows that "On My iPhone" is indeed a separate area called "File Provider Storage". You can add files to this area by dragging over the simulator. I don't think there is any way for a normal app to copy to this folder. So what I wound up doing for my app is to have my own picker for "built-in" files and have a separate command to use the UIDocumentPickerViewController to pick files that are not internal to the app.

Started happening to me too at the same time. DTS told me Apple just turned on throttling and to file a bug report about it. I want to make sure my code is waiting before submitting a bug.