This can’t work, because you are using a system font. System fonts have to be converted to an image during output to make sure, because nobody can be sure, that these fonts are installed on every visitor’s machine/device. Web design is not print design! You are not free in the choice of the fonts, you want to use.
To avoid converting font containers to images you have to use standard fonts or web fonts.
Because system fonts are converted to images, they obviously scale like images in responsive layouts according to the size of the browser window – that is, what happens in your sample site.
Please read my answer 4 in this thread for better understanding the issue: https://forums.adobe.com/thread/2357163
More information about web fonts in Muse:
By the way;: What the hell do you need a breakpoint at 7680 px for? Are you using a screen with the dimensions of a soccer field?
Thanks for clarifying that. My guess was it would behave like a font, not like an image. Looks like I have to use a web font then. I forgot to change the resolution before saving. The idea was to start designing for 8K UHD and scale down, but I figured I could optimise the site for it later, when the resolution becomes more of a standard.
You should google „logical/physical resolution“!
Just imagine: If you place images in „8K“ resolution, it has roundabout 10 MB!