craqgerbil

I can get this to work by just

/attribute @p minecraft:generic.max_health base set 10

which should set my max health to 5 hearts

/data get entity @p Health

shows I have 20 health 

now, I set up a dispenser with arrows hooked up to a pressure plate, should be 3 damage

It does the fake death, click respawn, everything back to normal, 5 hearts

this is on 1.16.4

edit this doesn't trigger when zombies attack and it seems like health is correctly 10

-I experienced this as well, I have a feeling it is happening when grass is checking to spread.
I put a dirt block down next to some grass, nothing initially happened, then a couple minutes later MC crashes-
edit- apparently it is the drowned causing this, I was making a mob farm with water. didn't even think about the drowned

Happened with 18w11a, loaded into 18w10d to salvage world. I wasn't working with any 18w11a features so mileage may vary.

This may have something to do with the random end gateways that show up in the outer lands starting 16w39a, maybe for them to be seen easier.