Hi Felix,
Sorry, but what you’re doing makes the typography configuration on your site complicated. This will cause more trouble than good.
You should not use custom CSS to apply general typography on Headings and Body, this will cause a lot of conflicts. Body and Headings general typography can be set on Theme Options > Typography. If you have a certain Headline that will have a specific font, you can do that at the element level.
Montserrat and Poppins are Google Fonts, you don’t need to upload that on your library (uploaded) and set up a custom font in the Font Manager. The same thing with Muli as you already have it pulling from Adobe Fonts.
The Revolution Slider has its own Typography configuration for its layers if those need to be configured. If not, then it can inherit the Body and Headings fonts.
Sorry if this is sound troublesome, but this will save you on a lot of typography issues, please reset your Typography configuration, remove the custom CSS that applies typography and configure the Font Manager not using the custom-font from your library, since those already available on Google Fonts and Adobe Fonts.
The real issue is the “can’t set a non-serif fallback” which I and Christian already reported on our issue tracker, regretfully there is nothing we can do about this for now.
Thank you for understanding,