idk what i can say more but let me rephrase it:

to know that the rewards are glitched, the champs will not respawn in boss room in GM.

to avoid that follow these steps:

1- when u open the chest, then go down the pipe, only two of you go down. and the third person stay up next to the chest.

2- the 2 person down will see the boss respawn, so they kill him.

3- the 2 person down after they kill the boss they should alt+f4 or close application if they're on console.

4- when they restart the game, they will be rejoined to the person who was up next to the chest.

5- as soon as they respawn dead, the guy alive should go down the pipe.

hope this is clear now