1) is this a known issue ? if so, how long has Tesla known about it
No, if they had known, they would have already fixed it.
2) is this vehicle specific ? If it's early production how many cars are affected ?
No, the early production reference was regarding the suspension/ ride quality and wind noise.
3) If it's early production as Elon said .... that means that at some point Tesla understood the braking had improved ... if so why was no fix implemented at that point.
He didn't say that. So this implication/ line of question has no basis.