A baby zombie, when pathing to a player, will freeze when it's under a 1 high space. I test this by placing a sheet of glass 1 space higher than the ground and watching see see how long it paths to me. They will go for a little while, then stop and it can take a long time before they start trying to path to you again for some reason. They will sometimes keep going more than 5 blocks but usually after 10 or so distance they will just stop.