Music Pauses While Screen Lock is Active (bug)

[demenece]

I'm using a Moto G with Android 5.1 and while the phone is locked (Black screen, immediate lock with power button) the music plays as expected. But when I press the power to reactivate the screen, the music pauses. And it stays paused until I complete the unlock process (no matter what unlock mode i use). Then it unpauses and plays along in normal behaviour.
The audio controls are displayed at the lock screens and the middle button changes from pause to play.
It's really annoying because I don't want my music to pause everytime I visit the lock screen to glance the time or notifications.

What can be causing it?