The exported MP4 is close to the timeline but not identical. Contrast and saturation shift a little depending on which player I use. How do I tell whether the export is wrong or the playback applications are managing color differently?
Compare in a color-managed environment and inspect the file's color tags. Players, browsers and OS display paths can interpret transfer characteristics differently.
Render a known test chart plus real footage and compare scopes as well as appearance. If encoded values/tags are correct but one player differs, the player path may be the problem.
Editing workflows
Do not compensate for one bad player by baking an arbitrary gamma change into the master. That can make the file wrong everywhere else.
Codecs & export