System notification chime volume remaining at zero after 'do not disturb'

[anon(3900582)]

I noticed that after having the phone on 'Do not disturb' mode for a few hours, the system notifications volume does not restore back to what it was and stays at zero. The other volumes are fine.
Does anybody have this problem?
Using a Galaxy note 3 with Android 5.0 lollipop.
Thanks