This one is driving me nuts. I open a fresh install of GP with BB and open a text module, go to advanced and change the device size and the fonts change. See screenshots 12 I tried it on a couple of different sites and they all do it. The only plugins I have installed are Beaver Builder Plugin Pro, GP Premium, Simple CSS, WordPress Importer.
When editing in a BB module if I change the device under the advanced tab (just like changing the device in the customizer) when the screen changes size the fonts change in the theme. If you look at the screenshots you can see the changed fonts. Screenshot 1 is before editing in the BB module and SS 2 is after.