Progress lost when saving

Hi, I have been working on a website design layout last night and when I finished editing in Upfront editor (Appearance->Customize) I clicked to Update the theme. The browser (Chrome on Win 10) window asked me if I should continue navigating away from the page or stay. This usually means something is going wrong and something is not saved (tell me if I'm wrong). I clicked Stay and Saved again. Unfortunately, the browser window came up again. As I knew I saved, I clicked Leave, hoping that it would be alright. Unfortunately, today I see that Builder does not remember anything of my work from last night and it has reset to a default layout.

How can I recover my work?

What am I doing wrong and how to prevent this from happening again?

This is the version I opened first today, I believe it was in the cache and thus it keeps my work from last night:

This is the layout version I get today after I cleaned the cache:

I suspect I'm not using the Builder correctly and probably should only work in the Builder (Upfront -> Builder) and not in Appearance -> Customize. If so, can you please confirm and explain the logic behind the two?

Thank you very much in advance.