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It would take quite a bit of video to “confirm” this, but crown of the unclean only attempts to apply def down to one target instead of all targets of an ultimate.
I use it on voresh, with over 200 accuracy and noticed this on goblins 3-1. Every wave he hits, buff immunity is applied on all and def down only applies on one of them. If I essentially have 100% debuff application success (which is the case based on the buff immunity debuff), it should have 3 chances to apply (multi-hit AoE) def 75% of the time on all targets. However, it only attempts this on the target of his basic attacks.