Using old/ or "new subtitle parser/renderer"?
(under rightclick > Options > Preferences > Subtitles)
If not using new subtitle engine, try whether problem is solved if you check that option!
If not solved:
(Also there: is "Draw subtitles to Overlay surface" selected?)
What is "Rendering mode" used?
(under rightclick > Options > Preferences > Video > [Video rendering])
(Did you already try a different mode?)