I've just confirmed that this error does not occur in iOS 14.3 or 15.0 beta simulators; it seems to be isolated to iOS 14.5.

What is the real technical reason(s) why non-macOS platforms don't support nested frameworks? Being able to supported nested frameworks in iOS & tvOS would make my life as a framework developer a lot easier.

I'm successfully adding the .dSYM to the XCFramework, but are the .bcsymbolmap files needed?

Agreed. This is very strange.