Found the problem thanks to the jstest tool.
The default state of the buttons was ON because of bad wiring on the switches.
I connected the TOP pin and the LOWER pin. It is supposed to be the TOP pin and the MIDDLE pin, in contrary to what the drawing shows on the switch.
For reference, this is CORRECT connection: https://www.dropbox.com/s/qhx5fd9r0k2vbq4/IMG_20170923_0439526.jpg?dl=0
And this was the WRONG connection: https://www.dropbox.com/s/bld45u4u2mm4gfc/IMG_20170923_0440122.jpg?dl=0