....So Miss Canon is calling Miss Kenko a dirty slut!
For what it's worth, when I use the Kenko 1.4x on my Canon 400mm F/5.6 L prime they hold hands willingly (must be lesbians?) and Miss Canon Seven Heaven 7D2 even goes as far as allowing enough contacts to touch so that all the AF pt options are available.
I suspect that it might have something to do with the 100-400mm being a zoom lens in which the widest aperture very happily slides as the focal length increases. [Note that I resisted adding sexual innuendo on this last sentence]