Hello,
I understand your concern but I seriously doubt that you have to worry about that.
If you are indeed paranoid and would like to "make sure"... then you can make a rip (re-encode) of the files. There are plenty of free tools around to rip media files. One that comes to mind is handbrake, a classic tool for re-encoding media files. https://handbrake.fr/
You can run the files through handbrake, set the quality etc to same as it is if you like, then have fresh video files created by you instead of the originals.
But as I said I dont think you should have to worry too much about it. But better safe then sorry I suppose
