My samsung behold cannot turn off the screen after lock button is pressed

Hi everyone, I just got this trouble with my phone.
Normally I press the lock button on  my phone, the screen is dim and after a few seconds the screen is off.

However, when I open the music player, and press the lock button, the screen does not turn off and it is only dim.  The screen may keep the dimed state forever, I have to turn off the phone and the screen is normal (without opening music player)

I thought it was a bug of the phone, but it does not occur to video player.
Is there config value for that ? or is it a software bug ?

 pls. help, thanks