As of writing the button with label _Add is displayed as _Add, this seems to be an issue with the SDK's CI not having current libadwaita main branch in which use-underline is set to True by default on the button.
As of writing the button with label _Add is displayed as _Add, this seems to be an issue with the SDK's CI not having current libadwaita main branch in which use-underline is set to True by default on the button.