This sounds very much like problems that occur when playing back any interframe compressed format like h264/mp4/... at anything else other than regular forward playback at normal speed.
Our main use case for these type files is that they're handed to you at the last minute and need to go on screen right now. At this point, you can load in this file, trigger it, and it will start playing back as expected. Promotor is happy, which means you're happy, which means we're happy, so everybody's happy.
The problems arise when you leave this file in and start shuttling, bpm syncing or really doing anything else with it other what you would do when watching a video online on youtube. This breaks playback and the file gets stuck (if you want to get technical, it's because you're asking the file to display an P or B frame, without having loaded the corresponding I frame first). If you leave it on pause for a bit, then resume normal playback, it should pick up and unstick itself. This is also why playing the file in a different player sometimes gets it unstuck.
We're aware of the problem, and we agree that the file shouldn't get stuck. But you should really not use these files for this purpose. That's not what they're meant for, and it will always be less than ideal. It took FCP/Apple years to figure out a good workflow for editing interframe compressed formats and we have to solve the same problems, but in realtime.