If the problem goes away when you're on a tripod, then there's not a problem, period!I tried 6 RF lenses, and I was able to get two of them to replicate the problem for a time. But then they didn't. I then locked all of them down on a tripod and reshot the tests, and none of them show the problem.
This may be because:
1) I'm just not good at replicating the problem and am doing it wrong
2) We're all seeing shutter button mashing effect and attributing it to IBIS
3) The tripod is sensed and the problem is going away because the camera is turning off the problem due to it knowing it's on a tripod (although I deliberately rocked the camera the camera from startup to 1 second before testing so that it would minimize the odds of the camera detecting the tripod)
I've been shooting at 1/8th of a second for these latest tests to maximize the visible effect. Trying lenses with lens IS and without. All RF glass.
This just means that this whole thing is NOT a problem, but rather an issue with the motion of holding a camera and pressing the shutter button in fast enough FPS mode.
By the way, if IS is on then the IBIS stays on whether you're on a tripod or not. I rely on that all the time in case the tripod/camera vibrates due to other factors like wind or ground motion.