Kurast Undercity can sometimes get broken and it will not provide the upgraded rewards when playing in a group. It almost seem like it “forgets” that you had a Tribute activated and when you kill the boss, you will not get the rewards that the tribute should have given.
I was running a few Tributes of Radiance (United) in a group and we experienced a few cases where we did not get the correct rewards. At one point, we also noticed that the actual issue probably happens already during the run (possibly when zoning to the next floor or something like that). Usually, the map screen (tab) show the details of the active Tribute, but it seems that sometimes the Tribute info goes missing. It was quite clear from that point onwards that whenever the map screen stopped showing the active Tribute info, it always meant that there were no Upgraded rewards from that Tribute dropping after killing the boss (only a few legendaries).