Official comment
Before you added the first playlist, you had your entire collection selected.
After you created the first playlist, you selected it. Then you collapsed the folder containing the playlist but the playlist remained selected.
After creating the second playlist, you did not select it as you did the first playlist which is why the first playlist remained selected.
It's not technically a bug. You just want new playlists to be automatically selected/highlighted. I agree with that logic. This is a feature request and I will forward it along to our engineering team. Thanks for pointing it out!