The effect is not triggered in ranged combat (throwing weapon) only in melee combat, but the description does not say otherwise:
https://baldursgate3.wiki.fextralife.com/Fly+TrapEither the effect is buggy or the description must explicitly mention that Fly Trap is only available in melee combat.
In general, all melee weapons with effects should have a note in the description that the effect is also only available for melee and not as a throwing weapon if it is intended to be so...
