After reding your message I checked better, as I bought all caps with focal lengths numbers, except one, with a skull graphic, which went to the 40 Art (as there was no 40mm cap in the lineup); I tried it on my other "free" RF lens the 16 STM (the 50 STM as you saw got a 50mm cap that was fitting)...and surprise! My skull cap doesn't fit, too! I tried all my other "focal lengths"caps (85, 24-70 and 70-200, that were only installed on my EF) on the RF lenses, and they all perfectly fit both on RF 16 and RF 50.
So it's just the non-focal-length that doesn't fit RF...but you have bought just that type of cap unfortunately; I'm sorry I didn't do the extended test before, but after I mounted the 50mm cap on the RF 50, and it was fitting, I imagined all of them were fitting; looking inside the two caps, the three screw "spacers" are exactly in the same positions, but the skull cap has all spacers which looks a bit thicker, while all the other caps have the three spacers that looks way thinner, so they probably slips on the RF mount for a fraction of a millimetre better.
I'm so sorry for that, misguiding you; but from what I've tested, the "focal lengths" caps were, and are, working.
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