wondering if by some chance the source material is from a screen recording or smartphone. If so, your sources may have a variable frame rate which can be problematic to begin with and definitely might cause issues with exactly where your edits happen. Also, what do you mean by "modified in some way?"
tell us your source properties and the camera that you shot with. Do your source properties match your sequence settings? Any reason you can't continue working in premiere 2019 until you finish the project?
Be sure to delete your cache files when you switch versions. For me it has become mandatory since 2019.
• Start PP from it's normal desktop icon (IE: don't open from a project file link). • Close the Open/Home screen (and you will be in PP without a project open). • Go into Edit/Preferences/Media Cache (Mac users adjust menu name) • Then Press: Remove Media Cache Files: and Delete... • Select: Delete all media cache files from the system and OK • Close PP - It will create any needed cache files on the fly as you edit.