I've been using VLC for a while now, and I notice this in every version:
I put a TV show season in the playlist (20-25 videos, 40 mins each). After a while, when using the mouse wheel to turn the volume up and down, the white volume bar on the right side of the window stops appearing.
I haven't managed to find a rule (like, after 5 videos, it stops showing) and I'm thinking it might not even be related to the number of items in the playlist, but maybe to how often I switch from full screen to window size, or maybe related to switching windows (Alt + Tab). I noticed this never happens when playing the first video in the playlist.