Bug with PTA and or Trinity force.

I was practicing and trying different builds with , one of them was a crit build. When I got the dummy low it started dropping below 100 health when it shouldn't, and then it died after a few more hits. This build seems to be the only one that kills it, + 80 AD it doesn't matter what other items you get as long as you have 360 AD or more and you have a , I also took PTA for my keystone. I tried to use other keystones but PTA seems the be the only that allows me to kill the dummy. I haven't tried this with other champs.