Pretty much what it says.
Headlines can only be update inline. They cannot be update from the “Edit text” modal / dialog window.
This is true for both preexisting headlines and new headlines. It is the same for same sub headlines.
I did manage once or twice but tried about 20 times (using Firefox and experiencing the same in Chrome if that helps) (I have cleared the Pro cache)
Sorry I seem to be having the problem with text elements as well. Can only do inline editing which is problematic as I am wanting to use HTML to add .
EDIT it seems that if I wait several seconds before closing the window then the text does update. It seems to be that it only takes effect if I leave the dialog window open until the text updates in the preview. If I close it before then the update does not take effect. This slows things down a lot.