I am not sure when this changed, but a year ago, when Insert Copyright symbol was created, (TM) was replaced by ™. Now it is replaced by :trade_mark:. In the composer preview, I still see: 
Which is how it used to work.
But in the published post, (:trade_mark:) is shown. The emoji is always black, regardless of the background, so the contrast is problematic when you are using a dark color palette. I wonder what the advantage of using the emoji is.