Migrating my code from 0.8.0 to 0.9.0 it seems that love.joystickpressed is called for indexes starting at 1 while Joystick.isDown expects and index starting from 0. Minimal code:
Yeah, it slipped through the cracks unfortunately.
But it's already been fixed for the next version (probably 0.9.1), and the wiki documentation for [wiki]Joystick:isDown[/wiki] has a notice about it.