I had a pretty quick response from Kenko about these issues:
Dear Mr. Dustin Abbott,
Thank you for your inquiry to Kenko International Division.
We are so sorry for this late reply.
We are sorry to hear your negative experience with KENKO TELEPLUS PRO 300 DGX series.
We have recognized some Conon Lens with EOS Mk III or EOS6D does not work properly, and they are still under investigation.
The combination with some lenses might work, but we have determined to ask customers to refrain from using KENKO TELEPLUS PRO 300 DGX series with Mk III or 6D.
We are sorry for your inconvenience.
Kenko International Division
I would assume that they aren't going to stop selling tele-converters, so I wouldn't be surprised to see them release updated versions of the teles not too far down the road. I wonder if this is a Digic 5 issue?