Steps to reproduce:

• Create a custom spellcasting mob,
• remove brain goals, flee_sun and restrict_sun
• set entity_attributes > generic_follow_range to a high number
• The mob still will not approach you from the distance set in the config.

I have tested this issue in Magic 10.8.2,
The mobs contain the attribute data in their NBT and these tags are correct, any non-magical mob with these attributes will attack the player over this distance, but magic mobs require the player to come closer.
My assumption is that something in the magic mobs AI is still being overridden and does not make use of this distance.