Hearthpwn

Maybe they are in the process of updating the feature and will be releasing a better version soon?  Or it's a bug.

Did some digging, and this is on the known bugs report:

• [Added 6/3] [Collection] The team is aware that “Complete My Deck” is not currently functioning. This is expected to be fixed in the next major patch.

No idea if the next major patch means tomorrow after the expansion announcement or when the new expansion launches in about a month to month and a half.