I can confirm this bug. Just recently lost/recaptured a colony of mine, loosing 2 brigades. Unfortunately, I got sloppy with the backups, so I can´t provied one before it happened.
It doesn´t seem to have changed from earlier versions.
Every combat turn, my colony would take 1 point of damage, which doesn´t destroy anything and nothing else would happen.
Once I received 2 points of damage, resulting in a loss of 0.025 colonists, some infrastructure and a mine or a factory, the population surrender event triggers.
In my older games, this only ever happened once in every game / with every new database.
Maybe the guardians are not flaged as active by Aurora until they capture their first colony or something and so Aurora finds only one race on a planet, fighting itself.
Edit: Posted in the bug forum