Go to the first keyframe closest to 26 seconds. Is there a mask at that keyframe? If there isn't a mask, then that's why all the video frames before 26 secs. (approx) are blurred. I don't know if that is how it should work or not. You would think that when you reach a keyframe that has a mask, that the mask will be there at least until the next keyframe. Zoom in on those keyframes so you can check for "hidden" keyframes that are only a few frames apart. Maybe you accidentally Reset the mask on one of them.