Ronald Hamelik

I am using MP4Joiner and MP4Splitter versions 3.8 . and using the Window 10 Movies & TV" program to view videos. If I take an MP4 video which plays OK in the app and split it, the split videos play OK as well. But if I join these same short videos back together, the resultant file will not open in the app. What information is being kept when splitting but not renewed when joining? I love both tools but as an old programmer, this would be unacceptable to me. Can you fix it?