For some reason incendiary arrows isn’t working for archers for the Abbasid. Camel archer and crossbowmen get them but not regular archers. Maybe the composite bow upgrade gets rid of the incendiary arrows.
If you upgrade fast reload first then incendiary arrows, it overwrites it and fast reload is gone. If you do it the other way, the incendiary arrow upgrade is gone from Archers but you get the faster reload.