Noto Sans Siddham in recent versions have GPOS 'dist' after 'mark', which for non-HarfBuzz engines means any attachments can become separated. In this particular case it seemed like 'dist' could just as easily be the first lookup without breaking anything, maybe this is a sanity check that could be applied?
I defer to @simoncozens on if this can be a fb check but it seems plausible :)
A font developer wrote me privately that
I defer to @simoncozens on if this can be a fb check but it seems plausible :)