strange thing with iphone8

hi all,
I got my new iphone8 on Friday and it's really cool. I just noticed something strange which i don't know whether it's a problem or just a normal thing.
you know when you toggle the mute switch button on and off, when it's muted you should feel a vibration. sometimes when the screen is locked, It doesn't vibrate, but I think it mutes the sound, then it vibrates when i unlock the screen. I personally don't think it's a hardware issue as it's working very well when the screen is unlocked. has anyone encountered such an issue?
thanks