When standing on most blocks, including a lit campfire, an armadillo will roll up when near an undead, or a sprinting player.
When standing on either magma, or a cactus, it does not roll up under any circumstance.
Comments 2
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I can reproduce this on 1.21.11-pre3. They also won’t roll up when taking damage from powdered snow, and will actually unroll themselves if they start freezing while already rolled up