Why this happens is that the keymap there is the one for the MIME
buttons, not the article commands, in spite of outside the button
(`C-h c E' there tells that the `E' key is bound to the command
`gnus-mime-view-part-internally'), but there is no handle data
associated with the button above.

What provides the MIME buttons keymap there?  There is neither a
text property nor an overlay that provides a local map, so the
keymap used there should be that for the article commands.