If you look at how ships in the bubble are moving you will notice that they always look alongside the course while their trajectory being a wavy line so what all ships shuffle their positions with time.
That means the possibility of any logistics ships to found themselves in a position closest to the border of the bubble.
And this is where the fast ships are supposed to attack.
And by what reason are the other ships within the bubble so far away that they can't simply consolidate once an approaching threat has been detected? If we're using the logic of the fleet as represented by the fleet bubble, why would it then necessarily be all the logistics ships separated from all the main combat ships that the attackers encounter first?
Since they actually can go around the defending fleet, they also fully capable of picking exactly that location.
While defender can do nothing about that because all its ships are stuck on the single course. Ordering one group of ships to change it means detaching them from the main fleet.
And that's impossible. Sad also.
Ships can travel in any direction regardless of orientation since there's no drag or air resistance to slow them down (except in nebulas, arguably) - heading in the direction of movement is only necessary for acceleration and course changes. Ships can move towards one another, further shortening the distance between any ship(s) even close to being isolated, and "the rest of the fleet" as if they weren't already contiguous.
The fleet bubbles evidently leave enough room for two fleets travelling in opposite directions to stop or slow down to combat speeds before overshooting one another. Changing the disposition of a fleet's ships towards an incoming force is accomplished within the space/time between the fleets "meeting" (ie getting so close they're forced to take action; committing to the battle or disengaging) and the initiation of combat.
Once both fleets have met, there's nothing stopping the reserve/non-combat ships from simply falling back behind their combat ships once the attackers are committed to a given direction of approach.
Being able to detach (an) element(s) of your fleet is something I've wanted in the game for a long while, and this is where the "cut-off/regroup" battle type would make the most sense - but it'd only be able to be initiated when a fleet is already separated, and the composition of that detachment would be decided by the player/NPC. Otherwise, there's no reasonable way to justify a weaker force successfuly dislodging any given ship or group thereof from the rest of its fleet, without engaging in direct combat.