...having just purchased a fire resistant safe they recommend NOT storing your ammo inside it.
Understood. However, it might be a good idea to post a "hazard" sticker on the door, indicating that black powder is stored inside. Such notice provokes second thoughts in those who would try to torch it open.
I think the dealer we got it from was recommending not putting the ammo/powder in the safe because in the off chance that the fire temp/duration exceeds the safety rating, the stuff may ignite.