My use case is currently "special", since framework B, statically linked with framework A, is also being used by the app importing framework A (This is done like that because Xcode does not currently support linking dependencies dynamically unless explicitly specified in the package manifest file). But this is another issue, discussed in other threads, so I don't want to deviate. In this case I am assuming that I need library evolution for framework B too.
After some research it looks like it was possible before to enable library evolution using swift unsafe flags,
but it looks unsupported.
Thanks for the help!