That random walking of e.g. passive mobs is disabled outside of a 32 block radius around a player is probably done for performance. But it causes issues with monsters tracking players if their "generic.followRange" attribute is increased. This can at most be set to 2048, but e.g. a zombie summoned with …

/summon zombie ~ ~ ~ {Attributes:[

]}

… does not seek out a player that is 50 blocks away. Getting within the 32 block distance and then moving away again does keep the zombie focused on the player, but it does not start tracking outside of that radius.