I am missing several card backs. Unsure when it occurred but I am now missing the Heroic Naxxramas card back
This is a known issue. See Known Issues List 2019 - Updated Dec 12th
This was supposed to have been fixed in the last patch:
- Fixed an issue where Card Backs earned from completing a Heroic Adventure would not show up properly in the Collection Manager.
Are you sure you’ve experienced this after the update? Is anybody else still having the issue?