Same issue here. My app with widgets works perfectly fine when compiled with Xcode 15, but when compiled with Xcode 16, widgets no longer work on iOS 15 (they continue to work fine on iOS 17 and 18, not sure about iOS 16). When trying to run on an actual iOS 15 device, there are loads of error messages in the console, including a crash with "Swift entry point not found".

I have a very similar issue, also with just a ProgressView in a circular widget. It’s not showing anything, at all. When using on device preview, after rotating the device a few times it starts to show, but is completely messed up… hope this gets fixed soon