Comment 3 for bug 1604900

This bug effects me too.
Just upgraded on of the machines from ubuntu-mate 15.10 to 16.04. It now does not shutdown. Always reboots instead.
The way to stop it from rebooting is to push a power button while on the grub screen.

PS offtop: Dear Martin Wimpress, thank you so much for your efforts and podcasts (bot JB and Ubuntu podcast)! Love hearing from you every time!