This solution is rather a workaround, but the main problem is solved.

When you add a WordPress Style Variation it is possible to add a new font. However in the style editor it seems that the new font cannot be enabled. When you save, the font is automaticly changed back to the original font.

There is a solution, however cumbersome:

Change the font in the template. There it’s possible to use the new font. Above you see a menu with a custom font.

Of course this behavior strange, and does not make sense.

PS: the font features are improved from version 6.5.4 and up. Now you can add additional fonts from the FSE in the main design panel.