@Grapi is right! AOSP is still using the very outdated vCard 2.0 format and it won't ever change for obvious reasons. Google does not have anything to gain enhancing the default Contacts app while at the same time it tries to push their own replacement apps (Google Contacts, so on so forth for everything else like Keyboard, Messaging, etc.)
Fossify, as well as other modern contact replacement apps, use at least vCard 3.0 and even vCard 4.0 for the better ones like Fossify. Obviously, you can not import a vCard 4.0 file into an app which only understands vCard 2.0 at best.