I try to describe the problem, you may have a better idea:

The bypass method e.g. used on the PI works as follows. video player never calls AddVideoPicture and AddOverlay only if one has to be processed. FlipPage is called on every cycle in order to keep the GUI responsive. FlipPage won't consume a buffer if nothing was added. The problem was when a subtitle was on screen and subtitles have been disabled in the GUI, the subtitle did not disappear. This has solved the issue. Setting this flag makes FlipPage consume a buffer and the sub can be cleared on next cycle.