LESS compilation of SSX may sometimes generate invalid CSS with missing colors

This happened after modifying the SSX of the menu application, here is the step that led to it, but I have not try to reproduce:

1) change the SSX
2) clear the LESS cache
3) load the SSX (using the URL of it)
4) found that @dropdown-link-hover-bg; was translated to nothing, seeing invalid CSS rule like background-color:; in the generated CSS from the MenuMacro SSX.

I know this report is currently partial, and I will complete with further findings if any.