Also related, but targets a standard 5' reach: When you grapple an enemy, where are they?
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them. The target of your grapple must be no more than one size larger than you, and it must be within your reach. [...]
Escaping a Grapple.
A grappled creature can use its action to escape [...] The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
Moving a Grappled Creature.
When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.
Based off of the information for grappling, there is no reason for the grappled target to be forced to be adjacent to the grappler, nor any requirement for the grappled target to be adjacent to the grappler to begin with, only that the target is initially within the grappler's reach.
I see a number of things that need to be addressed:
Can the grappler move you back and forth within their reach?
Can they only move you closer/further when they move?
Are you stuck at the same distance that you were at when they initially grappled you?
Anyone have any information on this?