What is your OS?
What
rendering mode do you use?
(rightclick >) Options > Preferences > Video > [Video rendering]
(If you're not using
"EVR (Enhanced Video Renderer)" selection in
[Subtitle properties] of
"Enable drawing subtitles under image with EVR renderer" has no effect)
Disable "
Draw subtitles to Overlay surface" in
[Subtitle properties].
If that doesn't help perhaps also read
this post.