Good noticing Lubi!
You are in fact correct in the instruction you are following – it is considered less safe for the knee-joint to extend past the ankle.
The word is that when Bikram was questioned about this he said “That’s the way I do triangle – it is a question of body geometry.”
So there you have it. Keep doing it your way and you’ll be fine – aim for that lovely triangle shape (like Rajashree’s straight line on the RHS) and a vertical shin … there you have it.
Namaste,
Robert