We are having the same problem with video on Moodle 3.7.1 where videos will not play in the Moodle Mobil app or in Safari on an iPads. The same videos will play fine on a Moodle 3.6.3 site. The video format is not the issue. These two sites have identical media settings. However, they are hosted by two different services. I feel it is either server settings or version differences that are the cause of the problem. We have entered a support case with the hosting service to help determine the cause. I will report their findings when we get some more information.