[HarfBuzz] I wonder why glyph 'uni0647' is not substituted with the one at 0xFEE9('uni06BE.fina') in Tahoma font

[Su Scott]

I tested harfbuzz.ng by a sample string: "همهٔهمهٔ" 
string = L"\u0647\u0645\u0647\u0654\u0647\u0645\u0647\u0654"; 

a sample pic by MS Word and an output file both in the attachment...

I wonder why glyph 'uni0647' is not substituted with the one at 0xFEE9('uni06BE.fina') in Tahoma font

................................................................................................................one mapping record of shaping table in harfbuzz.ng :
{0xFEEB, 0xFEEC, 0xFEEA, 0xFEE9}, /* U+0647 ARABIC LETTER HEH */

in GSUB Table :
<Substitution in="uni0647" out="uni06BE.fina"/> 
................................................................................................................

Many Thanks if any clues to fix it.

Scott


 		 	   		  
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