Sounds like u have some corruption in your original file.
Also, NEVER save over ur original file, always “save as” when revising a source…that should be standard practice.
When changing things in qt7 you always have to “save as”
For example, if u trim a 1gb (10min) file down to just a few seconds and simply save, it will still be a gig, u have to save as to trim the size down.