I helped develop some Android based software in the past, it seems the DX-50 might be running on a 4.0 platform and I do have experience with that. Without video evidence of exactly what happened prior to and after the event, there isn't squat anyone can do to help diagnose the problem. Play the Sherlock Holmes card and try to gather those who had an instant jump from low to high output together and see if they had anything in common. What batch is their unit from, what type of file was playing, was the unit paused, did they properly upgrade the firmware, are they near any strong wi fi signals, did they get something sticky into the volume button without knowing, could the battery be faulty and hiccuping voltage, did you accidentally push another button while raising the volume, or vice-versa, has the player been dropped or tampered with at all, are there any loose jacks or buttons on the unit, did the screen flicker or dim prior to or after the freeze? With some of these questions definitively answered, I ( or some other devs I know ) might be able to confirm it is a hardware problem or the software. If it is a software problem, then it is just in need of an update. If it is a hardware problem...well that is not good.
Based on what I know about the general platform this DX-50 was built on, there is no power attenuator or dialing system hard coded in a way like a volume knob would be. You can't just crank it up to the max output in an instant, it has to be gradual or else it won't register with the software. I do not know what iBasso did in their design but the removal of such an important safety device such as " enabling the device to instantly jump up to max volume just like a pc volume would do with the click of a mouse button " seems like something that would not be overlooked. I could be wrong. Without seeing the problem first hand, all I can do is sit and wait for someone to post a video of the entire event so I can see it, or wait for iBasso to respond and confirm this dreadful problem.
If that kind of code would have to be removed, I'm thinking corporate espionage or a vindictive coder.
Another poster here earlier said it was indeed gradual, but quick. That jives with the failsafe you mentioned.