The Communications integrations in 4.3 require some code in the theme to work.

I would hope that the moodle.org theme is a child of Boost to avoid bugs such as this one in general, but for some reason in this case the code appears not to be here, so I guess the moodle.org theme overrides some of the templates.

You can get a clue to this from the related fix for Workplace themes on https://tracker.moodle.org/browse/WP-4636. It's probably quite a quick patch.

I'd really appreciate this being fixed soon so that we can implement our integrations between the community forums here and the community chats on Matrix.