I'll check to see if it's the case with altplatforms' bug, but I've discovered that it's having Jetpack's commenting feature "Let readers use WordPress.com, Twitter, Facebook, or Google+ accounts to comment" activated that prevents the Webmention Form from appearing on the page. If I deactivate it, it not only appears properly, but shows up twice with the standalone plugin installed.
Incidentally it also causes issues on my 2016 theme where if one types too much in the enclosed field, the UI moves down and it becomes impossible to actually post the comment!
While I'm thinking about it, as an Indieweb-forward thought, would it be possible to put the Webmention Form at the top of the comment section instead of underneath it? While I'm (and I'm sure most of us are) receptive to comments in general, it would be nice to encourage others to own theirs at the top and then provide the default comment form underneath it. Just a thought...
I'll do some additional testing and report the bug to JetPack shortly.