QMAKE_TARGET_BUNDLE_PREFIX is ignored -> signing errors

The fix in QTBUG-65673 seems to be at the wrong place. Instead it might be better placed in default_post.prf instead.

Since the code is executed in default_pre, QMAKE_TARGET_BUNDLE_PREFIX is always set to the default value calculated by qmake using PlistBuddy and cannot be overwritten by the user's actual .pro file.

A workaround to this is manually setting 

xcode_product_bundle_identifier_setting.value = "com.mycompany.myapp"

in the .pro file.