In my creative world, I found a totally indestructible enderman wearing a backpack. Resistance V gives monsters 100% damage immunity (20% per level). That should be capped at level 4.

Latest.log file (includes the mob's NBT string gathered with /data): https://gist.github.com/TheDeviantCrafter/4a76b3e27239452eadbe996bbb463607

Screenshots:

fixed in latest release