From my understanding, follow_range is supposed to dictate when an entity can detect a player, and will follow the player until it fails to check for the player in the given range, however Piglin Brutes seem to ignore that. They will always detect from the same radius around them (thus never being able to "look" further than, nor be limited below, 16 blocks for players). It seems they use follow_range as the max amount of steps they will allow themselves to take when advancing in any direction. As a few examples that I found;

1. When follow_range was set to 0, the Piglin Brute refused to move at all. It became completely stationary, however still aggroed onto the player when the player got within the default detection range by playing its angered noise and holding its axe in the air.

2. When follow_range was set to 2, the Piglin Brute moved 2 blocks at a time. When I entered its default aggro range (much further than 2), it became angry at me but would pause every 2 blocks for a brief moment before moving another 2 blocks. 

3. When follow_range was set to 32/64/100/1000, the Piglin Brute followed what seemed to be its normal AI pathing (likely because it didn't need to take that many 'steps' to get to me).

Factors such as dimension, other NBT data, world type, datapacks, etc. don't seem to affect it.