As we grow older our teeth and gums deteriorate. When I was young I used to be able to make labiodental plosives, [p̪ b̪] — but I can’t now. I can no longer get a hermetic seal between my lower lip and upper teeth: there’s always some escape of air through the gaps between my teeth.
This is no doubt one of the reasons that no language has distinctive labiodental stops.
It means that when I try to make [ɱ], as in information or emphasis, I end up with some sort of [ṽ]. Inevitably some air escapes through the mouth.
What this is leading up to is that I used to think that I would usually pronounce seventy as [ˈseɱnti] (with the alveolar nasal somewhat dubious). I am sure plenty of other people still do. But what I now say is [ˈseṽnti]. And that must have been an intermediate stage in my previous nasal-assimilated pronunciation.
Underlyingly it’s still ˈsevənti, and always has been.