What browser? I know Froala has some issues with Firefox but forget if this is one. I have not seen any new ones since updating to 2.3.7 though, just the ones that I already knew about
I encountered this on my site as well as here on XF.
When you go to reply to a post with a smilie DO NOT click into the editor but just click the smilie icon in the menu and post one, and you will see there is an extra line break above it. It does not get posted however.