I had a fleet of 550,000 jumpcruisers orbiting a planet with minimal defenses. I split off 500,000 and ordered them to leave the system, and then ordered an attack on the planet before the fleet departed. At next watch the attack progress message over the planet recorded 90% casualties for the attack, even though the fleet that jumped out of the system had not actually been destroyed (it had simply left orbit).

If it does not do so already, the game should save the initial combatant fleets' strengths at the beginning of the battle and calculate the progress messages based on this number, adding or subtracting arriving or departing fleets from it in order to accurately represent the state of battles that get joined or partially fled from by fleets after the initial battle order has given.