When baby zombies come to attack me, they hit me a few times while looking up at my face. After a few hits, they'll just hold still right next to me, looking straight as if the don't see me. They won't notice me or try to attack me again until I move a little bit. This has been happening since 1.9 but I never got around to report the bug until today. It's happened in all versions of 1.9 accept 1.9.3 because I didn't play that version while it was out.