Viewport meta tags disappear when 'Use layout's stylesheet(s)' is unchecked

[tickletheory]

Hi, I'm encountering a baffling but frustrating problem.

I'm using Custom CSS and I always uncheck Use layout's stylesheet(s) when styling layouts to avoid duplicating css. I've tried this on a few themes, and every time I do this, the meta tag for responsive mobile disappears.

<meta name="viewport" content="width=device-width, initial-scale=1.0"/>

just stops being rendered in the HTML.

Is there any way to put this back whenever I use custom CSS, without keeping Use layout's stylesheet(s) checked?