Hello, @Oliver Theiß,
Sorry for the inconvenience, but that is not the bug, the option you indicate in your provided screenshot is not for mobile panel settings but it is only for the zooming purpose for different devices, by choosing this option you can see live, that how it look likes on different devices: https://postimg.cc/K1cFQqhh
If you want to change the Font on the mobile version only, you should have to use Custom CSS for it.
Regards 8Themes Team.