See #4227 (comment):

you resolve as much as you can at each step but % can't be resolved at computed value time unless they are a terminal type for the property. If lengths have to wait.

So it's a bug if the implementation assumes that 0% <= 50% <= 100% and doesn't wait until the percentages can be resolved into a length. This is already implied in the spec, https://drafts.csswg.org/css-values/#calc-simplification

If root is an operator node that’s not one of the calc-operator nodes, and all of its children are numeric values with enough information to computed the operation root represents, return the result of running root’s operation using its children, expressed in the result’s canonical unit.

I think reading the spec to the letter, we should never simplify percentages. However that's a bit of a hard one to fix with properties that only parse percentages because that requires preserving the calculation a lot longer than browsers are now.

Hah, I mid-aired with oriol :)

But properties that don't resolve percentages into another (non-number) type, like opacity, compare percentages as percentages. So you can resolve immediately.

No, @emilio is right - the spec (mistakenly) never simplifies %s in min()/max(). It assumes they'll turn into another type, at which point they can simplify.

I should fix that to specify that %s which would resolve to another type in the context shouldn't simplify away in this case, but %s which stand on their own can.

That said, the OP is indeed invalid - the spec definitely does specify that you don't resolve a min() or max() in that case until simplification can change the %s into lengths, at which point you get the correct values.

I did just now add a small note making the behavior of percentages clearer, tho.

@tabatkins

This was indeed specified in the spec, and I just missed it. The spec makes sense as it is, but the added note is really helpful.
Thank you for clearing this up :)