NOTE: There’s kind of a “Surge” portal that can appear (from what I’ve seen, randomly) and will take you to fight another boss. With my first character, after killing Diablo the first time, nothing happened, but after repeating for the second reward, the portal appeared, we used it and took us to Skarn, killed it, no more portals and when exiting, I had a reward to claim, appearing as a Task from 6/4 that wasn’t there before (or that I don’t remember seeing):
With my second character, again after the second Diablo round, the portal appeared, but this time, after the second and third bosses, the portal kept appearing, to take us to the next, so we killed all 4 bosses (as in what should seemingly be the “Event Gauntlet”).
Not sure why I got it completed after only 2 bosses with my first character and had to go through all 4 with the second… But it’s DI, from Blizzard/NetEase, so it doesn’t come as a surprise to find this kind of “weird behaviour”, actually.
UPDATE: Third character, killed Diablo, no portal. Killed it for the second time, no portal either. I had thought that maybe the “Gauntlet” would become available after having killed all 4 bosses at least once, but it doesn’t seem to be like that. BTW, the task that I mentioned as “hidden” is nowhere to be found in this 3rd character. Like I said… Not surprising that weird things keep happening in this game 
UPDATE#2: After 4 tries, finally got the “hidden” reward. Started with Butcher, portal took us to Diablo, next to Skarn and… No more portals, but after exiting the reward was there to be claimed. So… 2 bosses, 4 bosses, 3 bosses. This “gauntlet” thing makes no sense 