[Wii U] Erratic input management on most cores

Description

On most cores (everyone I've tried besides FB Alpha and Mame), input management is very erratic on Wii U.

• the only working controller is the Wii U Gamepad, and only if it is set as port 1 input
• even when "all users control menu" is on, it does not work most of the time. For instance if my Wii U Pro Controller is connected before opening retroarch, and even if I set it as port 2 input, it can't control the menu. If I connect it after opening Retroarch, it can, until I change inputs, then even if I come back to the Wii U Pro Controller, it can't control the menu anymore.
• The Gamecube Controller (Mayflash USB adapter) seems to consistently be able to control the menu though
• If I set something other than the Wii U Gamepad as port 1 input, then I can't control anything anymore, sometimes not even the menu (I mean : even the Wii U Gamepad can't control the menu anymore)
• Weirdly enough, hotkeys work fine in game. E.g. if I set the Wii U Pro controller as port 1 input and bind some hotkeys on it, game inputs are not recorded but hotkeys are.

Expected behavior

• Controllers set up as port 1 input should be able to control the game
• All controllers connected and setup as port x input for some x, should be able to control the menu, once "all users control menu" is setup.

Actual behavior

See description.

Steps to reproduce the bug

1. Open RetroArch, change port 1 input from Wii U Gamepad to something else

Bisect Results

Happened since as long as I can remember, even though I haven't been testing intermediate builds since around a year ago and until now.

Version/Commit

Latest nightlies.

• RetroArch: latest nightly.

Environment information

Wii U.

Start by reproducing the controller and menu behavior on Wii U using the latest nightly, following the listed port and connection-order steps. Trace the platform's input handling and menu-control path; done means port 1 controllers control games and all configured controllers can control the menu when that option is enabled.