This issue has also extended to draft formats - the card box is off center right:

The game starts in windowed mode, whatever option was set before closing the game the last time. I press ALT+ENTER to make it full screen. When matchmaking starts, the screen automatically goes back to windowed mode, I press ALT+ENTER to put it back to full screen. When matchmaking finds an opponent, the game goes back to windowed. I press ALT+ENTER and it goes back to full screen. When the match ends, the game goes back to windowed mode. Please fix this annoying bug ASAP. I'm on a 3440x1440 and the black bars on the sides are fine for me, but not the windowed mode which also cuts off part of the game screen at the bottom.

In addition to the game reverting to windowed mode, there is not a native resolution that fits the 21:9 landscape of the monitor. Full screen works and properly creates a letterbox, but the setting does not save properly.