In my view this isn't a bug. One of the things Pharaoh does well is having different cultures and rosters have different approaches to warfare, as reflected by them having different unit abilities and attributes available to them.
If an Aegean or Mesopotamian player really wants to set things on fire, there's native units you can gain by conquering Lowlands for Hittite Archers, Fenkhu for Light & Armoured Syrian Archers, Sinai for Shasu Archers, etc.
As such, I don't believe there's a need to give that capability to those factions.
All the Best,
Welsh Dragon.
The updates for Total War Pharaoh Dynasties have been excellent; however, one of the things that players most want and expect is a solution to the issue with Aegean and Mesopotamian archers who don't have the option to shoot flaming arrows, and infantry troops who can't burn buildings with torches.