Plz fix. Thanks. Have a nice week. 
That’s not a bug. Diamond cards which are not part of the reward track or achievements can be disenchanted now.
I bought Diamond Yogg-Saron with the Miniset and I can’t disenchant him.
You recieve all the golden cards if you purchase the golden version of the mini set and you will also recieve a golden Yogg.
The diamond cards from the mini sets are “just” a bonus for purchasing the golden mini set. That’s why you can’t disenchant them because you got them for “free”.