A CSS declaration placed inside {% comment %} can appear in the generated stylesheet.
Reproduction steps
Looks like if you have a comment tag with a colon inside of it like this:
{% comment %}
Mobile: symmetric outer columns so the centered logo stays centered regardless of the
desktop menu position setting. Desktop layout is applied at the md breakpoint below.
{% endcomment %}
I can confirm that this is a limitation but it’s something that we have a theme check for already: StaticStylesheetAndJavascriptTags
Disabling this check is not recommended, as using Liquid inside {% stylesheet %} or {% javascript %} tags can lead to errors in your theme.
If you want comments in your CSS and JS inside of {% stylesheet %} or {% javascript %} tags I’d recommend using those languages native comment syntax: /* ... */.
Does that help clarify things? Were you already aware of this limitation but hoping for some kind of platform change?