Clicking on the button to display all recipients of a message doesn't do anything, in the kmail 5.2 header style. Reproducible: Always
I think that you have kmail version with security fix which breaks it. I fixed it in 5.3.2
No, this is with vanilla 16.08.2, no patches in messagelib or kdepim. It just wasn't available from the drop-down versions list.
Ok so it's fixed in 5.4.0.