Van Punch cannon drop (Borderlands 3)

Hello, I had such a problem, today I decided to get a Van Punch cannon and I went to kill the boss, the boss didn’t get the necessary legendary, but a friend’s good legendary fell out and I took it and I decide to look on the Internet why I didn’t get it and I found where other people the same problem and they recommended to restart, but the problem is that I picked up the gun that I dropped out, I want to know this boss changes every time after the restart or did I need to pick up anything?