This is a nice new feature!
However, personally, I would prefer if the default were set to either “Discard” or “Cancel.”
IMHO, if the user opts to click “Cancel” when closing the card layout window, it is relatively safe to assume they know what they are doing and actually don’t want to save any changes they made to the template. Defaulting to “Save” on the confirmation pop-up feels a bit counterintuitive and unwieldy.
Also, I suspect it might confuse people who are used to the previous behavior, leading them to, for example, press Esc → Enter out of muscle memory, ending up saving unwanted changes.
In this light, it seems like defaulting to “Cancel” could be the safest option, as it would force the user to stop and think about what they actually want to do.