For some people, a 50% "deficit" fails. And the entire concept of "X workout burns Y calories" is completely bunk. Again, there have been multiple studies to this effect.
It's not physically possible for a 50% deficit to fail, what you probably mean is that their energy expenditure was incorrectly estimated at +50%.
Are you actually saying some people don’t lose weight on a 50% caloric deficit? Is there any evidence of that?