Amumu passive Bug

Was testing out amumu on the practice tool and ran into a bug. used his ult on 3 dummies causing a curse mark to be shown on top of all 3. had my W on but only the middle one i bandaged to first was getting hit by true damage. despite having the green curse mark on top of the heads of the other two. when you click on them, there is no curse mark debuff on them which explains why they arent taking the true damage. The strange thing is, this only happens when i use rylais and landry's. the bug doesn't happen all the time but every so often there would be curse marks on the dummy's head, but it isnt taking any true damage or have the debuff on them.