My flexibility is constantly being improved upon with my Taekwon-Do training and nothing has hindered that by being on T. I'm not only a martial artist, I'm a cyclist, so my legs are beasts! I'm currently trying to limit lower body workouts in hopes of letting my top half catch up! So muscle development doesn't matter. Look at Jean Claude Van Damme, he can do insane splits beyond 180 degrees and he's got legs that are beasts. Scott Adkins is another fine example. He's an all over tank with insane flexibility.
I'm fairly sure flexibility only dissapates when you're not working on it to at least maintain. I can't see T making a difference unless you work hard to bulk up your legs while not working on flexibility. But then the same would still happen if you weren't taking T, just possibly at a lesser extent.