via #4310 ...

https://drafts.csswg.org/css-flexbox/#min-size-auto

Nonetheless, although this may require an additional layout pass to re-resolve percentages in some cases, this value (like the min-content, max-content, and fit-content values defined in [CSS-SIZING-3]) does not prevent the resolution of percentage sizes within the item.

I think this parenthetical is trying to say that min-size: min-content|max-content|fit-content do not prevent the resolution of percentage sizes same as min-size: auto. But I can't find where this is specced?

(Also none of the browsers support these keywords on min-block-size. But they parse it.)