Subtitle text appears correct in Captions window but not in preview or final video

Hoping someone has an idea to resolve a problem I have with trying to do some subtitles in Sinhala. 

The text appears correctly in the Captions Window, but in the Preview and in any video output some characters aren't composed correctly (sample files attached). I have tried multiple Unicode Sinhala fonts and they all have the same problem.

Have tried turning ligatures on and off (in both the track settings and the general preferences) and that makes no difference.  I believe the issue is with the text engine, but that's no longer in preferences, so I can't change anything there. The text works fine in Indesign, but if I change the paragraph composer there back to just Adobe Paragraph Composer rather than "World Ready" composer a similar problem occurs.

Thanks in advance for any help!