Nope, it's a bug with Kenko. Complain to Kenko. properly.
Complaints have been made, and they already deprecate the tc with 5d3/6d because of this bug... no way to "fix" it than to replace the whole thing since 3rd party manufacturers (Sigma, Yongnuo) only recently got smart enough to put usb interfaces into their products.Clearly did not reverse-engineer Canon's software properly.
I doubt that, they re'ed what was the lens-body protocol at the time - but Canon changed or extended it, at least to enable dual afma for zooms. No one knows the change necessarily meant breaking the competition's products, but personally I don't think Canon minds a lot as they sell their own tcs.
I don't think so. The version II TCs were out before the 6D/5DIII and did not need to be revised when the 6D/5DIII came out. Canon extended the protocol, but what was existing worked as before. That Kenko did not get reverse-engineer the software correctly is not Canon's fault. That is the risk of buying third party parts. My original post is that it is not Canon's fault that Kenko does not work with new cameras. Kenko could have updated the software/firmware for a nominal fee, but chose not to. Not Canon's fault. Dgatwood's point is that it's Canon's responsibility to have their equipment work with third party parts. I'm saying it's the other way. What Sigma is doing is a step in the correct direction.