Oriental Striking Dummy randomly becomes invulnurable

[Viviza]

Time: 4pm central.
Frequency: About 25% of the time when striking it with any attack from any job.
World name: Ultros
Character name: Viviza Viza
NPC name: -
Monster name: Striking Dummy (I guess)
Class/Level: The dummy is level 1. I've tested it as Monk 50 and Black Mage 60.
Party or solo: Solo
In-game time: ~10pm
Area and coordinates: Mist Ward 1
Steps:
1. Just attack it. It doesn't matter if the attack is a spell, ability, weapon skill, or auto-attack. It doesn't matter if the attack is single target or an area effect. Occasionally the following will come up in the combat log: "The striking dummy is invulnerable and takes no damage."

EDIT: It only seems to do this if it's in close proximity to other dummies. The dummy's targeting circle will flicker red (in combat) and then back to yellow (out of combat) and only appear on the aggro list for a split second.
EDIT2: I've tested this with other dummies, and they don't do this. It's only the oriental striking dummy.

Small example of my combat log while playing as monk:
You hit the striking dummy for 204 damage.
You use Bootshine.
 The striking dummy takes 331 damage.
 You move into Raptor Form.
The striking dummy is invulnerable and takes no damage.
You use Twin Snakes.
 The striking dummy takes 303 (+28%) damage.
 You gain the effect of Twin Snakes.
 You move into Coeurl Form.
The striking dummy is invulnerable and takes no damage.
You use Demolish.
 Invulnerable! The striking dummy takes no damage.
You lose the effect of Coeurl Form.
You lose the effect of Twin Snakes.
You hit the striking dummy for 200 damage.
The striking dummy is invulnerable and takes no damage.






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