Positional forms of ain are missing unicodes.

[ibo-o]

When /arain (ع) is added to a font its positional forms come without their Unicode.

ع arain	      U+0639 (Unicode definition: ARABIC LETTER AIN)
  arain.init  Unicode is missing
  arain.medi  Unicode is missing
  arain.fina  Unicode is missing

However, there is another set of /ain that has their Unicode in GB's list.

ﻉ ain.isol U+0FEC9 (ARABIC LETTER AIN ISOLATED FORM)
ﻋ ain.init U+0FECA (ARABIC LETTER AIN INITIAL FORM)
ﻌ ain.medi U+0FECB (ARABIC LETTER AIN MEDIAL FORM)
ﻊ ain.fina U+0FECC (ARABIC LETTER AIN FINAL FORM)

I think either arain should be renamed as ain and the suffixed ones [arain.init/medi/fina] should be removed from the list. Or [ain.isol/init/medi/fina] should be renamed as arain and merged with existing ones.

Since the other Arabic letters don't start with ar first option sounds more convenient to me.