Steps to reproduce: get attacked with 1 cp in the middle fleet, get it killed.

Re-engage, game remembers your fleet placement, enabling you to have your entire fleet in third lane without the required 1 cp in the mid.

savefile before the attack: battleAutosave.sav

I rather expected the game to run a check before you click okay. At least this (hopefully) doesn't break anything, but I've send your save and report along.

Common "meta" practice nowadays is to sacrifice single cp ship on mid line in favour of bulldozing a selected lane of your opponent (preferably the third). It's mostly a rock-paper-scissors move. Where you expect to fight only a 1/3 of opponent's fleet.

This is especially strong against behemoths (stuck in mid lane cause of the check) which somehow do not exactly shoot back at the third lane effectively, while the third lane kills them.