I've just come back from a days shooting, mostly video, with my R5 and Canon EF 100-400mm II L lens with the Canon EF-RF adapter (the plain one).
I've had a really odd thing happening when in video mode and having IBIS on (Camera(7)->IS (Image stabilizer) mode ->Digital IS->on, not enhanced), the LCD panel preview (but not the physical camera/lens itself - this was on a tripod!) was jumping up and down violently and the image shown seemed to lag as well - so when moving the camera I was getting both shaking and ghosting of images that should have been only on the screen a few seconds earlier. I'm not talking about a little shake here, but violent - perhaps up to 1/2 of the field of view, and at the refresh rate of the LCD panel. This was only in the preview mode mind, so starting to record seemed to calm the camera to the point that it was useable, but of course it was impossible to frame and preview what I was going to shoot. Setting stabilization->off seemed to prevent this happening, which is why I presume its IBIS related, even though I can't reconcile the ghosting as being a simple IBIS issue.
This is only in video mode - nothing odd was seen in photo mode at all. I got home and checked this wasn't a lens issue by putting my 24-105 RF lens on the body and the shaking in the viewfinder was still present. I was resigned to getting a repair done, but then I looked for a 'reset' function on the camera and found one in the menu system under wrench(5)->reset camera->basic settings, so I ran that and the problem seems to have gone away - hopefully for good!
Bizarre, or what! Anyone seen anything similar?!