[see my next post for possible solution]
Hey,
Couldn't sleep, so I've had a quick look at this. I'm on Unity 5.3.3f1 and Playmaker 1.80 f40.
It will be of no help, but I get exactly the same behaviour as you.
The highlighted colour works until you press the button. After the press the highlighted coloured sticks, even when you are not over the button and only returns to normal when you click somewhere else on the screen.
I installed Playmaker UGui prefab, so that's not the issue.
I wouldn't have noticed this as I only use buttons with images, and they don't highlight when hovered over (mine is a mobile game).
My guess, and as I said, my friend, I am an idiot and there are much smarter people on the forum than me, that it's not a Playmaker issue, it is either a Unity issue or a Unity setting.
I deleted all trace of Playmaker from the project, so I was left with only a canvas and a button, and the same issue is seen
Hope that helps, at least a little
Now, to get some sleep!