Another thing I've noticed even in ARR fates is that sometimes enemies spawn so far away from their own radius that if you attack them as soon as they spawn they'll immeditly become invincible while they run to their retreat-to point.
Ugh, noticed this too, always makes me groan every time.

FATE mobs should never pop outside of the blue circle. This is really the only thing that needs fixing, rather than messing with level sync and how it works (the whole "leave sync on until disabled by the player" suggestion is just asking for trouble and complaints)

Whether this means moving the spawns or making the FATE circle bigger (the latter is probably simpler to do from a design standpoint) doesn't much matter to me.