Copy variation definitely occurs, it’s more common in consumer lenses than L series lenses, but certainly occurs in the latter. For example, check out Bryan’s review of the EF 24-70/2.8L II. He ended up testing four copies of that lens because the first two did not perform well. When I was reviewing the EF-M 18-150 for TDP, my results differed substantially from Bryan‘s posted ISO 12233 chart images. He ordered another copy and tested it, found it was significantly better, and replaced those images in the tool.

I did test my RF 1.4x and 2x with the RF 100-500, and the results were as expected, so I don’t think there’s an issue with my RF 2x. In real world shooting, I’m not sure those differences would be evident anyway. But I do definitely recommend testing a new lens when you get it.