Because of limited features in GOS contacts (not supporting birthdays ...) I installed the Fossify Contacts. I imported contacts into internal device memory.
Thing is: It seem that PhoneApp (GOS) can access it, but not the birthday apps (used BirdthdayDroid or Birday). I can add contacts/birthdays from this app, and I can read thme via contact apps, but - birthday apps seem not be able to read other contacts/birthdays (the imported once). Although they all seems to be located in "internal device". What's wrong here?
Also strange. I stopped GOS phone, contacts and Fossify contacts - deleted all data but contact data seems to survive that. Couldn't find a way to delete them all.
Of course I gave contacts, Fossify contacts and phone permissions to access contacts.