As the title says, however the slider next to it for "Minus frame height" does function properly.

Here I set the width to 20 and the height to 40. As shown on the mob to the right, the height is correct, but the width is not:

If I change the width to 200 and the height to 20, the height did change accordingly, but again the width is being ignored:

That's not a minus frame, it's just nameless. Nameless frames use the minus height but normal width.