I just saved an existing Muse file to the latest version (2018) and all my responsive layouts/breakpoints disappeared. Of course it's the one time I didn't make a backup copy. Why did they disappear and is there any way to get them back?
Not sure, but you may have a look here:
This may sound different in your language, though.
it's the one time I didn't make a backup copy
the 2018 build should auto-save a backup when you convert a older .muse... did you go back in and delete it?