I just updated to iOS 12.1.4 (iPhone 6s) and now my VLC 3.1.5 will not play audio for the next track if the screen sleeps.
I press the home button to wake the screen and I see the VLC monitor showing it is playing (no audio yet) and after about a second or two the audio will start playing again (not from the beginning of the track but from where the player says it is playing silently).
If I keep using the phone and don't let it sleep the player will move from track to track playing fine. I used this yesterday fine, pre 12.1.4 update.