I noticed this problem when I made a survival world in 15w41b but I won't be going back to see when this bug first appeared and decided to wait a while to see if anyone would report it and seeing now that 1.9 is getting close to being released and still no one has reported this bug. I decided to make this report to get it fixed before the release. So in 1.8 when you used an invisibility potion, these guardians would not target you. In 1.9 snapshots they do target you which makes working on a guardian farm a lot more difficult.