I am unsure what the issue really is .... it's maybe the lens' ability to focus rapidly enough or something else in the electronics of the lens that was changed between version I and subsequent versions. Seems unlikely to be a camera firmware problem to me but I am far from an expert on this type of issue.

I don't think it would deter me from getting an R5 because the electronic shutter is still a good option for fast action.

Another thought would be to wait until there are good prices on the used market for a 1DX III which has the rapid frame rate but significantly lower resolution.