How is this not a bug, given what's indicated in the documentation regarding if and the evaluation of expression syntax? To wit:

When you use expressions in an if conditional, you may omit the expression syntax (${{ }}) because GitHub automatically evaluates the if conditional as an expression. For more information about if conditionals, see "Workflow syntax for GitHub Actions."
https://docs.github.com/en/actions/learn-github-actions/expressions#about-expressions

No ${{ ... }} at all should work, too. But using it for parts of the if value will always create a non-empty string, which is always true.

Let me just say, before continuing, that I'm very grateful to you for sharing this knowledge.

These combinations did not work for me:
if: ${{ success() }} && ${{ matrix.category == 'foo' }}
and, as you've mentioned, any combination that involves enclosing one clause in ${{ }}, but not the other. I haven't tested an expression that does not group clauses with ( ) (ie. ${{ success() && matrix.category == 'foo' }}

I think either the parser needs to be improved or I should edit the docs, unless you know of a place in the documentation that states explicitly the rules that you've shared.

EDIT:
My initial response was incorrect: if: success() && matrix.category == 'foo' does work.