Researching ‘Incendiary Arrows’ Technology from the University/Madrasa Building updates Archer/Crossbowman units to use a different weapon stat(extra damage and flame arrow visuals). While this is intended behavior, this currently overwrites Imperial Age Elite Crossbowman’s weapon model(a bigger crossbow made of iron) back down to Castle Age weapon model(a wooden crossbow).
This currently applies to all civilizations as far as I am aware.
This is the Elite Crossbowman without the Incendiary Arrows upgrade, you can clearly see the iron crossbow weapon model. This is reverted to Castle Age weapon model when the upgrade is researched.
Similar bugs exist when combined with the ‘Composite Bows’ Unique Abbasids Technology. If you research ‘Composite Bows’ after ‘Incendiary Arrows’, the new composite bow weapon model overwrites the incendiary arrow and the Archer unit no longer has flame arrow visuals while firing.
On the same token, if ‘Incendiary Arrows’ is researched after ‘Composite Bows’, the Archer unit no longer uses the composite bow weapon model and reverts back to Imperial Age bow model and flame arrow visuals.
Similar bug exists for the Abbasid Camel Archer unique unit as well. When you upgrade the Camel Archer to Elite status and then research ‘Incendiary Arrows’, the Camel Archer no longer has the bow weapon model in their hands and it is instead stuck on the ground, which looks pretty ridiculous.
The Camel Archer’s weapon stats also get bugged, which has been reported before. After researching ‘Incendiary Arrows’, the Camel Archer unit loses its bonus damage against Light Melee Infantry as seen here.