Video is attached, but I will also provide reproduction steps:
Step 1: Enter B43 on practice mode, and set fleet stats to 3.125
Step 2: select mods as follows: railgun fighter, missile armor frigate, armor cruiser, regular heavy cruiser
Step 3: Place ships as follows: [fighter on a3, a5] [heavy cruiser on a2, a4] [frigate on b5] [cruiser on b1]
Step 4: Ensure that a1 is empty, and there are only two groups of fighters placed
Step 5: place a fighter on a1
Step 6: Start the battle. the fight should play out with the hull gatling surviving for a long time, killing the heavy cruisers, and the fight should end in victory with 96.75 FR used
Step 7: reset the fight, and start the battle again. The hull gatling should die early, allowing the heavy cruisers to survive and end the fight with only 88.54 FR used
The decisive shot comes from the armored cruiser, but the cruiser's targeting is affected by how the enemy skirmishers die later, and it misses its chance to target the gatling.
Funnily enough, if you do this to the other fighters the bug presumably still activates, but it has no effect on the outcome of the battle. If you create a set of "tainted" fighters on another slot, then swap them into a1, the bug will cause the 96.75 clear to happen.
I thought at first that the bug was caused by placing a ship over another ship, but through testing I've narrowed it down to just being about whether the fighter loaded into the grid (from a previous clear layout or from resetting), or if it's newly placed. A newly placed fighter in a1 on this setup will cause a different outcome from a loaded in fighter.