Invincible spider without hitbox

I had been on a far trip. I came back to my base and got hurt by something. i ran into my base, note that there was a one block gap, and i kept getting hurt. once i died it said i got killed by a spider. it was invincible and had no hitbox. otherwise it could't have gotten in thru the door.

I came in to my base and all my stuff had dissapered. They couldn't have despawned in less than 10 seconds.  Plz help me with this bug.

/Hefahl