It is the Custom Headings element seems to be causing the issue on mobile. If I assign each one an extra class called footer-heading can I then use some CSS to hide on mobile only? I think that would work.
the good news is that the stacking behaviour is what they’re supposed to do but they’re not stacking properly due to some css errors that handle the phone sized layout.
however, as you’re using a paid for theme you’ll need to go back to the developer for technical support. it’s likely a very easy fix for them but it’s their code and they’re responsible for supporting it.