Big thanks to everyone for the additional details & reports in regard to this issue. Especially the videos 
We could finally reproduce the “browser back button” issue that wiped out the custom classes for @macksix and @aslotta, and the global class @digitalgravy added in his video above around the 20min mark.
After adjusting the Cache-Control HTTP header inside the builder, the issue no longer occurs for us locally. We’ll ship this fix in 1.6.2 in a few days (performing a few more tests in the meantime).
If anyone still encounters this issue in 1.6.2+ please reply to this post incl. a screenshot that shows your Response Headers inside the builder (as illustrated in the following screenshot).
Please also mention if you are referring to the loss of your global classes (the ones you can access via the dropdown) or the CSS classes under “STYLE > CSS > CSS classes”.
