[ES2]Why do my fleets insist on staying in one group?

Simple scenario - playing as Unfallen I'm seeding a random system. I have a basic fleet to defend the Vineships, consisting of two Wells, and two Needles (so 2x defence and 2x offence class). 4 pirate ships attacks me, including some kind of medium sized ship. Now my defending forces consist of 7 ships, adding to 7 CP.

However in the battle preparation screen I cannot do anything. Vineships insist on staying in the group 2, together with the rest. Nothing I could do to split them apart, and so I was forced to go in with my vulnerable ships at close range. I had some randomly found beam weaponry from a planet on the other side of the galaxy that was 3 tiers above what I had researched, so I thought that I would kill them anyway.

And I did. But only the 2 ships that were in the enemy middle group. The medium sized ship and the random straggler that were in the group 1, continued to shoot at my fleet for the remaining 80% of the battle, with my ships helplessly starting at them, because they are "close range", and apparently unable to do anything about it. I lost a single well and the battle ended up in a draw, but because I was blockading the system, and that battle was prompted by that, those fuckers attacked me again, this time killing everything but the Vineships, because again, the fucking larger ship was off to the side and my fleet was just staring at them.

The hell?

Edit: Apparently it must be 5CP in a single fleet. Still it makes no sense that they could just float out of range with my ships being set to "rush them as fast as you can" tactic. Also, a single Fireship (medium hull) and a single small ship armed with two short range beams is apparently capable of taking down 9 enemy ships, as long as they are not randomly out of range. Huh.