The problem I can´t solve is due to the final frame of the project animations I load. Since the last load it gets stuck in the final frame it loads and it can´t be reduced the time of the animation to make it shorter, it just stops the animation in the last keyframe and wait to reach the last frame of the animation, where there´s no keyframe, but instead, you can grow up the time of animation all you want with no problem and get it back providing that you don´t make it shorter than I was at the first load.

I made sure to have all bones selected to ensure that there´s not any of them that cause this, and I din´t find out someone who have the same problem.

Yeah, that was just the problem. I turned visibility off some time ago to bones at the end of the hierarchy that I used to do some specific animation I didn't even remember that, and with that bones with visibility off, the dopesheet didn´t notice anything about their keyframes.

Above that, thank you, sir for your help, that was no bug at all, so this thread should go to other section.