Well, I didn't even remember that we support sizing of image based subtitles. That's neat.
Looking at the code, the placement tries to preserve the center point of the rectangle. So a centered subtitle will stay centered, a subtitle under a certain character will stay there, etc.
Also, the code limits the amount of sizing so that it never grows outside the video. So I can't explain why it's falling off the edge in your screenshot. Would it be easy to provide a sample?
One note is that if the subtitle gets enlarged and was off center, it could get pushed to the center so it will fit.
As for putting subtitles outside the video, we're sometimes limited by the renderer. With EVR, it's almost impossible unless we switched subtitles to a floating window (instead of an overlay in the renderer), and that would hurt performance.