I don’t think it’s possible to disenchant core/reward cards. You probably disenchanted the version of this card from Ashes of Outlands. This isn’t an issue as long as it’s included in the core set. You probably still have the 2 regular / 2 golden versions from it.
Filter by Ashes of Outlands in the collection to find the version you disenchanted. This isn’t a bug if you chose to disenchant it.