I haven't tried a MergeText component for the entire Comment, but I know that when you are editing the Comment field, before exiting, I always have to click on the "Source" tab at the bottom left of the Comment editing window. If I leave it at "Design", any line breaks I have entered do not register and everything will flow into one line. FYI - The same thing happens when trying to edit the body (HTML content) of a Send Email component.
Try the 'Source' tab and see if it works.