Hello, this is a known bug. When holding a button on the Wii U GamePad that will trigger a mouse down event, for no reason it will toggle Down / Up.A small issue I'm having, when I try to send a mouse click, it often spams the click before holding.
I've gotten around it by just rebinding my mouse-click things in the games that need it, but I could see a few games that don't have that option.
No idea where to even begin helping you find out why I'd have this bug, (assuming it's not everyone getting it) so feel free to tell me what I should send your way to help. :v