[HarfBuzz] Arabic shaping issues

[TheSauce]

Hey,
I'm using harfbuzz with great results for all my Unicode rendering needs.
However I have one case where it's not working correctly, which is this
string: مِنْهُ
Specifically, character codes (in memory order): 1605, 1616, 1606, 1618,
1607, 1615
All my other arabic shaping works correctly, but this one ends up as the
attached image.
It looks correct in firefox, which I hear uses harfbuzz so I assume I'm
doing something wrong, but unsure what. I'm specifying 'ar' and the arabic
script enum for the shaping.
Any ideas what else I may be missing?
Thanks!
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