Just a heads-up; it happened to me recently for the second time: I was using the RF1.4x at a fairly high f-number, and I saw a big dark, fiber-like shadow that was large in the frame, and I told the R5 to do a sensor clean, and nothing changed. Then, I remembered that this happened to me once before, with an RF TC, and it was actually the body-side glass on the TC. So, I took off the TC, and sure enough, there was a smudge of something translucent on the TC glass.
Being so much closer to the sensor than EF TCs, RF TCs are able to image junk on the glass on the body side of the TC.