Please note: the original post mentioned the EOS R6 Mark II, that was obviously nonsense…
Canon released firmware ver. 1.1.0 for the Canon EF 200-400mm f/4L IS 1.4x lens.
Firmware Version 1.1.0 is incorporated with the following correction.
Corrects a phenomenon in which the subject may not be in focus on some occasions when using this lens with some camera models (*) and performing AF shooting from approximately 3m.
* Some camera models: EOS-1D X, Mark II, EOS-1D X, EOS-1D C, EOS 5Ds, EOS 5Ds R, EOS 5D Mark IV, EOS 5D Mark III
Download firmware 1.1.0 for the Canon EF 200-400mm f/4L IS Extender 1.4x.