I have a s21 Ultra phone, and a S8+ tablet. Both device are experiencing this issue. Whenever I add a relationship, either default or custom, it disappears when the contact is saved. There are other posts regarding this issue, and it appears a common problem. I was unable to reply to the other posts, so I have opened a new one here. Rhe only "solution" I have seen is that we have to delete all of our other apps one at a time until we hopefully find the problem goes away. Hardly the answer I would expect from Samsung on devices that cost as much as these. I hope to see a more proactive response from Samsung.
