So I import this part back into my project in order to merge them with the other parts I am finishing and rendering. The temporary solution I got is rendering each part I've already 'finished'.
It's disapointing, because I am trying to do a clean editing and I take so much time to do that. Sometimes the scene is from an other part of the same cropped video or it is a scene of a part I've already removed from my project. It's not an editing mistake, it just shows up. I've been extremely careful cropping the exact frames to make a good video, however, when I re-watch them, I see that at some points, for less than 1 second, the video shows an other scene which was not there. The more I work with those videos, more glitches I get. I'm splitting each video in many parts in order to merge certain scenes of them in just one video (at the same time, I am inserting a song in the background). It's hard to explain, so I will give you an example of what happened many times.
When I work with a considerable amount of parts of videos, they start to glitch in some frames. An annoying problem happens while I edit videos.