Slitherine

This has happened too many times to ignore. I was at war with the Gavakans. I attacked a planet with a fleet of 20 carriers with 400 fighters. The only fleet I saw on the planet was a Freighter. Since I had wiped out their fleet a few years earlier I don't think they had any real fleet. The message for a battle flashes. When I click on the message my fleet disappears. The message says all enemies destroyed. Instead my fleet was destroyed with no battle. This happened in my last game playing as the Koalas against the Magellans too. I can produce a save if that helps.

Yeah, this is an annoying bug. It has to do with the carriers being unarmed.

I can only speculate, but what I think is happening is that the game correctly determines that the enemy freighters are unarmed so it skips combat. It then goes to check which fleet to destroy, sees that your carriers are unarmed and destroys them even though they have a full fighter complement. You can fix this problem by adding 1 laser to your carriers so they no longer count as unarmed.