This was solved in a reply on Stackoverflow (https://stackoverflow.com/questions/75422505/how-to-prevent-xcode-from-adding-malformed-entitlements-to-my-dylibs/75422519#75422519), which seems to be written by ChatGPT of all things. The problems seems to have been that my dylibs didn't have any .entitlements files. I created new ones, and in the review shown by Xcode before submitting the app, it seems as if the entitlements in them were stripped, which makes sense if dylibs are not supposed to have entitlements. What makes less sense is that they need entitlements files.