I had this issue when I set my alarm sound as one of the songs on my Z10 and then I moved around some media on my SD card and it lost it's path to my sound. I'd change the tone and test then if it works change back to desired sound. A restart in the mix couldn't hurt just in case something is "stuck". If that doesn't work then maybe a bug.
YES, the screen is supposed to go white when alarm sounds and phone is in bedside mode. It lights up the room a bit and helps ya wake up and find the phone! You can also snooze by hitting anywhere on the screen, When bedside mode isn't used you must hit the small snooze bar and screen doesn't go white.