26.4.1 is a small server-side hotfix releasing today with back-end updates and the following player-facing updates and bug fixes:

• [Hearthstone] We’re temporarily disabling the “New Cards” button in the Collection, due to bugs it caused. We’re investigating a permanent solution to bring the button back in a later patch.
• [Hearthstone] Fixed a bug where decks that did not have an assigned card back were treated as having been assigned the “Classic” card back.
• [Hearthstone] Fixed a bug where Magatha, Bane of Music revealed all the cards she drew.
• [Battlegrounds] Fixed a bug where using Upbeat Duo on Putricide’s Creation would only grant the base Undead, not a copy of the creation.
• [Progression] Fixed a bug where the “Sun and Moon to Me” Achievement did not count the Core Set versions of the cards.