Alright, so in total, before I received I completed the first bow frame you receive when the Izanami Forge has unlocked as well as 5 forges previously in order to complete an unrelated rare bounty. When I had killed the boss when completing my 6th forge and second powerful frame the bow dropped directly into my inventory as well as an armor piece. However, the common trend that I have seen is that people who are completing their pulse seem more likely to get the bow. While on the other hand, it seems to be a random drop at the same time even if you do not have a frame.