This is buggish or off beat ux anyway. Dont you have to remove the combo class temporarily and then add the mobile class? I am quite new and have struggled with this, but there are other posts on the topic.
Maybe it seems logical if you have a non webflow background in css, but It really feels like you should be able to select the parent class and then switch to the mobile break of that parent class even if it doesnt exist.
Another recent post When styling in a mobile view, choosing a higher level selector changes the view to desktop