So, when a finger of the right hand impacts against a string, when using "tirando", the phalanges always bends. In this photo I show the right moment when the problem occurs:

I've been studying for 5-6 years now in a conservatoire here and always had that problem, only one professor noticed that problem. He commented me that he had that problem and the principal consequences of it was that the right hand would lack of the right strength and speed, this week I'm going to take classes with him to try to solve it. But in case it does not work properly I was wondering if anyone here had that problem.
I also noticed it is a somehow weird condition, i asked many fellow guitarist to show me their tirando technique and their phalanges practically don't move, so they don't have that problem. I also asked non-guitarist to try the tirando, and surprise, they don't have that issue either.
I tried anticipation excercises (again, I'm from Argentina, I don't know how you would call them), that would be: playing open strings with staccato and as soon as a finger attacks a string, the next one must be positioned instantly on the same string producing the staccato. It's an excercise a Matteo Carcassi specialist suggested me (he also suggested me to try the Carcassi treatise on guitar which didn't solve the problem).
I always try to trigger the movement of the fingers of the right hand by moving only the upper phalanges of it, but still the bottom phalanges bend.