I came across the exact same problem yesterday.
I found out that this pull request is fixing the issue: https://github.com/cebor/generator-angular/commit/c6d5ee67e108a0699f95c78702cb1939233226a8
What I did to fix it in my project was commenting out the following lines of code. According to the pull request this cssmin does nothing but replace relative paths by absolute paths.
// The following *-min tasks produce minified files in the dist folder
cssmin: {
options: {
root: '<%%= yeoman.app %>'
}
},
The a grunt build uses the correct path, which is "../fonts"