I'm not sure I'd consider that a bug, though yes the behavior is different than XF1. If you open the smilies, then it's reasonable for it to not be closed without you clicking the button to toggle it.
On here it looks innocuous, but if you have a lot of smilies, it just looks a bit wrong. I only mentioned it because I had members complaining.
No big deal I'll tell them they now have to close it manually, and how to do that, but some people - when confronted with change - (for the better) seem to want to find things to complain about.
I was wondering about this too. I have a lot of smilies and it looks odd that after you post, they are all showing yet. Hopefully that will be corrected...
I have had a few members get confused by this, they can't work out how to make the smiles go away.
Please is this something you could look into @Mike ?