BS.Player does not only show text-based subtitles (there are also text-based subtitle-files with a .sub extension; these have a small file-size), but it also shows graphic
(picture-based) subtitles like those of (substantially larger) .sub-files which (initially) form a pair with an .idx-companion.
Maybe verify under
(rightclick >) Options > Preferences > Filter management > [General]
Is "Enable advanced graph building" enabled? And "Allow intermediate filters"?
Another place to verify settings:
(rightclick >) Options > Preferences > Subtitles > [VobSub]