I guess that's kind of a clickbait title, it's not really a leak. It was just added to the digital downloads discretely, and it's been neat to read through to look for changes. This is mostly just the bestiary stuff, and I probably missed a lot, but slight spoilers for the second floor at the end.
Owlbears, Mimics, Dopplegangers, Web Lurkers, and Drow were removed. Mostly unsuprising, other than Dopplegangers, I thought their art was pretty distinct from D&D's, and the concept is based in myth. I'm on copium and think maybe it wasn't a good fit monster for a beginner's box, and they might still be in monster core. RIP Web Lurkers, I really liked you.
Giant Ants, Reefclaws, Minotaurs, Wargs, and Caligni, were added to the mini bestiary. Caligni have enough profiles put in to replace the multiple drow profiles missing, so I think it's the same number of monsters.
Basilisks, Bugbears, Ghouls, Gargoyles, Harpies, Orc Scrappers, Shadows, Wights Xulgaths, and Xulgath Bosses got new art.
Some mechanical changes I saw; animated armor got a longsword, ghoul trades paralyzing claws for sneak attack and stench, harpies no longer sing but get stench and a vacuum wind ability to suck people closer, shadow is now vulnerable when in light (and not specifically the light cantrip), skeletal giant traded their greatsword for a greataxe, the wight's drain life was renamed but seems the same.
It's mostly worth looking at for the cool new art. While some of the changes are interesting, it is hard to read too much into it because the beginner's box uses simplified rules. Ghosts don't mention spirit damage, for example, and I'm guessing hell hounds should be weak to holy in monster core.