If you find that MX Player is no longer behaving as it should then try the custom codecs from XDA, here is a short tutorial:
https://drive.google.com/file/d/1_YsoIdP9OUzVI2qN1cOxKGggInHEF1Ao/view?usp=sharing
https://drive.google.com/file/d/1_YsoIdP9OUzVI2qN1cOxKGggInHEF1Ao/view?usp=sharing