I've just created a new test project in X3c using the Normal template to see if I can recreate your issue.
I recorded a midi clip, double-clicked it to open the PRV, and I see no issue there (except for the annoying auto vertical zoom).
I saved and closed the project (without locking the tab or the screenset). I re-opened the project, no problem.
I closed the PRV (not minimize), double-clicked the midi clip, and I still see no issue (again, except for the auto vertical zoom).