Yes, what @jon wrote is the correct answer. The old kind of expression $node did always reference by looking for the same index. Meaning the expression worked for the first item, but as multiple workflows will be returned, will it fail for the second and all further ones. That will the resolve to undefined and so the first part of the path will be lost and so fail as the path is then invalid.
Ohhh, duh. I remember now, it has to do with the multiple iterations. Since the config node only has one item, it only works on the first item fetched from n8n. Cheers!