The output you're getting is intended. The only difference between a normal class and a silent class in Sass is that the selector for the silent class is not found in the generated CSS. Change your silent class to a normal class to see what's really happening:
.testing {
font-size: 1em;
}
.testing {
font-size: 4em;
}
.i-expect-4em {
@extend .testing;
}
Output:
.testing, .i-expect-4em {
font-size: 1em;
}
.testing, .i-expect-4em {
font-size: 4em;
}