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Describe the bug
Whether it is ranged, melee, drowning, traps, anything, rats who are not plagued does not take damage. They can't be tamed either. They hang around and attack any approaching player.
They appeared after a rat/flute invasion with the text "you hear the haunting melody of a flute growing louder".

To Reproduce
Wait out a rat invasion and attempt to kill the remaining rats without plague effect after the sun has risen. After the message "The mobs leave as the sun rises".

Expected behavior
The remaining rats without plague effect does not take any kind of damage no matter how it is dealt, and as a result, they don't die.

Screenshots
Well it's just a rat that doesn't take damage.

Info (please complete the following information):

• Modpack Version: 2.77.1
• Server or Singleplayer: singleplayer
• Version the world was created on: 2.76

Additional context
I haven't had acces to lava to test if that would kill them. Neither have i stacked up 128 blocks from sea level in attempt to despawn them faster. They are trapped under my base, so they are not exposed to sunlight, if that's of importance. I tried restarting the game to no avail.
They did eventually despawn, after waiting for one whole day/night cycle (~40 minutes).

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AlexModGuy/Rats#163