I am using MX5 v5.0.1.1200, and it seems this issue happened after some Windows 10 updates. (don't remember which one specifically)
Anyway as far as I know some apps don't have this problem in the latest Windows 10 such like Potplayer, which can choose file association during/after setup and won't get reset by the system, and it also appears in the default app list. So I think maybe it can be a good sample for your guys to figure out the solution.
Regards