I don’t know how to understand it 
www.loom.com/share/8977e3e1c26747e082896aed265ee7d0
P.s. i thing it’s about column flexbox layout inside which next row are pasted. Disabling column flexbox cures this.
Nice find! The collapsing layout is probably just a reality of such an advanced layout, but maybe we can help by adding a few styles only in the preview frame to enforce some generic column dimensions.
Update: We’ve made some adjustments here. 
Ok. I will test it in beta6 )
Fixed in beta6.