To entirely solve the issue of differences between the ones that cleared first and the ones that cleared later, without penalizing anyone, all they need to do is make individual chest rewards. After clearing, there's an X% chance that the player will get some gear, if they didn't clear yet.

In reality, this "hated" mechanic of not necessarily getting loot is what we already have, except for the fact that there is a gear that goes to SOMEONE. But it's still ultimately RNG on whether you get it. To make players feel better, have the chest always appear, let the player roll on it, but instead of rolling against other players, it would be rolling against the "computer".

There, problem solved. Whether you are in a party filled with people that cleared, or right after reset with everyone still having to clear, you have the exact same chances of getting the loot.