Invisible, unhittable mobs and invisible items after death and resawn. Re logging fixes it; you can then see them and have a chance to kill them. I Died three times to invisible baby zombies, twice to arches and 10 times other zombies. Recurring effect. The invisibility does not stop the mobs from hitting you! ouch.lol.
I gather this happened with a previous snapshot.
Otherwise the game seems to be running well. Definitely more lethal as expected.
The items that scatter from your body also become invisible but can still be picked up if you get near and are visible in inventory. That's not much help if the weapons you retrive do zero damage even on sword swing. lol.