We attached the epic link on the tickets successfully. And on the next day or just after that for a while, all the epic links disappeared from all the tickets automatically. We checked the logs but no one deleted them. We tried to re-attach the epic link to the tickets 2 times and they were gone again. Please check this issue for us. It doesn't happen with other projects.
Note: All the epics are still existing. And I checked the permission and project automation and they are all like other projects that I work on.
Thanks.