EDIT: This bug still exists in 1.14.4.

 

Increasing the follow range attribute does not work.

 

/summon minecraft:creeper ~ ~ ~ {Attributes:[

{Name:generic.followRange,Base:30}

]}

 

 

Using the above command(replace with whatever entity) does not actually increase the number of blocks at which mobs will aggro on to you. However, using a command such as:

 

/summon minecraft:creeper ~ ~ ~ {Attributes:[

{Name:generic.followRange,Base:10}

]}

 

...will decrease the number of blocks at which a mob will aggro you.

------------------------------------------------------------

Reproduction steps:

1. Summon any mob with a custom follow range number above the default
2. (  /summon minecraft:creeper ~ ~ ~ {Attributes:[ {Name:generic.followRange,Base:30}

   ]}  , as example)

3. Notice that the creeper/skeleton/spider/etc still will not aggro to you until ~16 blocks.

I have tested it with extreme numbers(in the hundreds,thousands,etc), the mob still does not aggro until ~16 blocks. As stated above, decreasing the value does work as intended, and mobs will not aggro until you are within the block radius of the custom value set.