Next track in the list is muted when playing a music album in the background

Playing a music album in the background (i.e. screen is blank) causes the next track in the list to be muted. The sound returns if you wake the screen up (to the lock screen). The lock screen music app shows that the track has still been playing, just without sound. The sound returns immediately on waking. It seems to always occur regardless of track length. I only noticed this after upgrading to iOS 12; not sure if it was this or a VLC update, though.

Also just tested in iOS 12.0.1 with same result.