In Fab, Fengtao refers to them as “Subpictures” and “Forced Subpicture”
Make sure that all these boxes are deselected. You can also turn them off in “Settings” so that none are preselected.
As my good friend SJ said, some subtitles are actually burnt into the image and cannot be removed and some forced subtitles are absolutely necessary such as in Dances With Wolves where I don’t think too many of us understand the Sioux language.
Signals has helped another user get this sorted, so there are always different requirements.