I think you have to choose the mobile view icon to make changes to the mobile view.
Say you are actively editing your computer screen view and you say, "how does this look on mobile?" and you resize your screen to 449 pixels wide. It doesn't look right, so you start adjusting the spacing and moving things. Then you open back up to 1920 pixels wide and it's a mess.
BUT, if you had said, "how does this look on mobile?" and then clicked the little phone icon on the upper right, it brings up the mobile view differently (maybe call it "mobile mode.") Make changes now, and when you go back to the computer screen view, it's all how you left it. In mobile mode, you can move things around and change the sizes, like making two different versions of your site. But if you are just resizing the browser window and editing, it's not capturing those edits as a separate variant of your site. It's interpreting them as changes to your main site.
I hope that helps, my apologies for the delay in getting back to you.