When the state of an sctask is changed to pending, closed complete, closed incomplete, closed cancelled, the Post button on the Additional Comments and Work notes is no longer available. When the state of an incident is changed to On hold, the Post button on the Additional Comments and Work notes is no longer available.
The issue is being seen because of the UI policies on these tables making the comments or work notes as mandatory under certain conditions, in this case, the value of the state field.
This behavior was logged as a bug in New York but the devs have explained that this is actually working as expected. They have explained that this is done to maintain the mandatory/required status of the comments or work notes field.
Removing the "Post" button is done intentionally. If you were to "Post" the comment, that would not fulfill the "required" requirement for the state field selection. To avoid this confusion, the "Post" button is removed.
They have explained that up until Madrid this was not functioning as required and has now been fixed to cause this behavior change.
As explained above, this is expected behavior now starting from NY - if the comments or work notes field is made mandatory using policies or client scripts, the Post button is removed from the Activity Stream