I'm really struggling to find any ancient Roman source which is definite about the use of flaming arrows.

Two web articles refer to the Romans using flaming arrows against elephants (a) at the battle of Beneventum against Pyrrhus, and (b) at the battle of Panormus against Hasdrubal, but when I checked the ancient sources I couldn't find any reference to flaming arrows at those battle.

I haven't checked the Greek sources.

Did anyone else find any ancient sources?

Is it possible that its just a myth. It certainly looks to be the (vary rare) exception rather than, as in TW, the rule.