Yep, I’m getting it here as well. It’ll start Plymouth boot screen & power button will blink orange, I press a key & it continues to login screen. I log in and it’ll do it again. Press another key and it then shows the desktop. Weird. If you or someone else finds out how to fix it, neat, otherwise it’s just a small annoyance (to me at least). The weird thing is, Ubuntu 10.10 didn’t do this at all, only Ubuntu MATE 15.04.
If your not scared to try this, there is a shortcut I’ve found to the shutdown problem. Right click on the desktop and click on Create Launcher. then choose Application in the first line, Type Shut Down in the name box. In the command box type shutdown -P now (do NOT add sudo to the front as it won’t work that way). Leave a comment in the Comment box & click on the Icon picture to find an appropriate shutdown icon. It should (if it works) kick you to the shutdown Plymouth screen & shut off.
I’ve found this works, but the drawback is there’s no warning box popping up giving you a chance to cancel, it goes right into the power down sequence. And if you want to have a restart icon, do the same except change the P to r.
Hope this helps.