Ah survivor ship bias I think it’s called, that’s one explanation, but how does the original gene mutation work ? How did it survive for apparently so many generations ? Seems to be the kind of mutation that would have not survived if it literally caused you to dance till you die.
The question is rather if there's sufficient evolutionary pressure to rule this out.
It led to a handful of people dancing till they die, ok, but maybe only in a dense urban setting which wasn't an issue before. Maybe that gene conferred other benefits, like more stamina.
Maybe it's like a lot of disease which don't seem to have benefits but still affect a much larger part of the population.