MKW with internal and external subtitles
 

Windows 7, internal renderer, x264 file, MKV container. Video file have internal subtitles and an external, same name in same folder. Using old subtitle renderer, there is no subtitle displayed, even if loaded (they show in subtitles list but not displayed nor show up in subtitle editor).
New subtitle renderer works fine.

LE: after going fullscreen, blackbars are somehow considered part of the movie resolution. If you position the subtitle right above the black bar, when exiting fullscreen, subtitle get positioned about 1/3 of the movie heigth and not at the bottom of the image.