I can confirm what your seeing. I played Mayhem yesterday for the Last Word quest part and when I checked this morning, it's now Breakthrough.
I'm guessing that people were having too much fun completing Crucible Bounties, so Bungie decided to put an immediate stop to it.
And I would like an explanation why this was changed with no notice too from Bungie.
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I got both parts (Kills-deaths and then medals) with my Void Titan. It took maybe 10 to 15 Mayhem rounds to get it all done. I just hung in the back and threw shields behind my rushing compatriots and picked off the ones who survived the onslaught of supers. Also used the Suppressor grenades (used chest piece that gave me double grenades) like a mofo. I just heaved them ahead of me, especially with blind corners or halways and then picked them off while they were trying to get their abilities up. Might have killed 6 people with a weapon :) My friends did it with their Hunter's Bladestorm and those Gauntles, but I had the quest line on my Titan so I went that route. Would have been faster the Bladestorm way.
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Yah. I noticed that afterwards. They changed it back. I managed to get 3 more runs in with my Hunter and Warlock right before re-set to fill some Hawthorn Clan Crucible bounties done in the last 20 minutes. I really wish Mayhem was permanent. I wouldn't mind a fun alternative to QuickPlay.