Did you by any chance enable the "Continuous AF"?
No it's off. Shooting one shot, AF servo, Center point.
When I use my 1Dx the lens snaps to the focus point then there is very little noise of the servo from the lens. On the R5 the lens appears to gain focus (green square) but there is a lot of noise from the lens as if it was jittering back and forth. I have had clear pictures from the R5 with this lens but very few. Most are extremely soft.
I've tried exactly the same settings between the 1Dx and the R5 and the 1Dx will beat it every time for sharpness.
Want to buy a 500mm Mk1