So, invicible brain slime is killing me. I disabled brain slime and it's still occur.
https://imgur.com/a/MjKB2

Do you have more information on when it happened? Just during a normal encounter? Were you on a server? It appears to be difficult to reproduce as I have not encountered it yet.

Did you disable it before it was invisible or was it invisible before you disabled it?

Visible brain slime spawn too(so invisible spawn when slime is enabled too). Single player. Just during a normal encounter.

Hoping 1.1.4 fixed this. If not, I will re-open.

I'm still finding these in 1.1.5c, it's still called a brain slime in the death message though, and i'm not sure if its invincible or not, I was in survival single-player at a desert beach.

I'm using 1.16_beta for Minecraft 1.12.2 and I also get killed by invisible brain slimes, but I still need to disable it to see if it keeps happening or not

I've just encountered this with 1.1.6 beta in MC 1.12.2 too. The first time, punching directly up seemed to knock it off as usual. The second time that didn't work for me, so I spam-punched in every direction which eventually worked.