Aaah, you’re coming from 2.1.4 
Unfortunately, that wasn’t clear from your report.
However, I can reproduce the problem. Apparently, it is necessary to set the root font size deliberately in the style manager, even though the placeholder shows the value from the theme styles (100%).
We will update this thread as soon as it is fixed.
Edit: After some trial and error, I can reproduce the problem even without updating – all in all, it’s confusing that the theme style HTML font size value is displayed as a placeholder in the style manager settings but is not used for calculations, unless explicitly set.