I’ve come across several posts on the Net that states that concatenation of MP4 files by means of MP4Box (from gpac) will make VLC play two videos concurrently instead of playing the videos in sequence.
I had the same problem, but once I had all the source MP4 videos generated with the exact same audio and video encoding parameters (that meant adding audio to the MP4 videos that did not have audio) things worked out fine.
Actually, I thought I had used the same parameters for all my MP4 files, but during the conversion from an AVCHD video to an MP4 by means of ffmpeg, the frame rate somehow went from 25 fps to 26.09 fps (you can check this using
ffmpeg -i somevideo.mp4 on all your MP4 videos). Once I added “-r 25” as the last option to ffmpeg, the issue was resolved.