Sigma has a USB lens dock that connects to the EF mount of the lens. I actually don't recall ever updating any Canon lens firmware in the past; didn't know it was possible.
I was growing more and more frustrated. I haven't set up many custom settings on the R5, and considering the R5 is still under warranty, I decided to tinker. I restored the R5 to factory settings and installed old firmware. Didn't work.
Reinstalled lens firmware and tried again on the "factory" R5 settings. Didn't work.
Upgraded R5 firmware to 1.3.1. Didn't work.
Upgraded to 1.4 firmware. Didn't work.
Tried one last reinstallation of the lens firmware. Suddenly the lens is playing nicely with the R5.






Based on that sequence of events, I have to believe the lens firmware didn't install clean for whatever reason. I'm happy I don't have to get rid of this lens. I'm not sure how it compares with the RF 35 f/1.8, but I've always felt the 35Art was the best value of all the 35mm offerings. Crisis averted.
Cool! An issue with the lens FW update was one of the best-case scenarios for what you had described! Glad it was fixed with a reflashing (I'm knocking on wood so as not to jinx it).



