We hadn't added this rule before because there was no such rule in
scss-lint. Instead, we were following it without a linter, and so we
unintentionally broke it sometimes.
But now we're using Stylelint, so we can add the rule and let the linter
check we're still following it.
One file was OK when we only had a couple of mixins, but recently we've
been adding quite a few.
We can now avoid a SCSS Lint warning by excluding just the file with the
affected mixin.