I didn't get the diamond Loatheb

[BigHugger-2468]

Yes, this is a known issue, mentioned in the “Known Issues” topic:

@GnomeSayin-11972

[Added 4/5]: The team is aware that the Diamond Loatheb card—a reward for owning a complete Curse of Naxxramas collection—is not claimable by players who already owned the entire Curse of Naxxramas set prior to this patch going live. The team is working on resolving this issue.