I'm not sure, where IS the shutter on an R or RP? Is it farther than 5mm from the sensor? Canon has patents for sensors that can take shutter-look photos without shutters. It's possible this lens simply wouldn't be compatible with RF cameras with shutters. Nikon and Canon both had lenses in the 70s that required the mirror to be locked up so they could get closer to the film, so there's precedent for certain lenses that don't work with all bodies.