From New York version, the post button should disappear when the journal input becomes mandatory. But this feature does not work as expected. Also, on inspecting the code in the browser console, we can see that the older code of the activity stream loads(compare it with any instance that does not have issue).
Release or Environment
The cause of the issue is the system property 'glide.ui.use_jelly_activity_stream' being set to false. When this property is false, it loads the older activity stream and the latest features will not work.
To resolve the issue, set the system property 'glide.ui.use_jelly_activity_stream' value to true.
activity_stream, post, mandatory, glide.ui.use_jelly_activity_stream