An independent people army that was embarked assaulted one of my cities, presumably by attacking the harbour. With units stationed in the city, and the garrison itself, I easily had enough to defend - but when I confirmed the battle, the enemy immediately won the fight and took the city without any rounds taking place.

I believe the issue is that they attacked my harbour from an angle such that the deployment zone for my forces were all ocean tiles, and since my units weren't embarked they couldn't spawn there. Therefore at the start of the battle none of my units were actually in the field, and the game recognized this is as the loss condition for the fight. I tested this by going back a turn and buying a ship to sit in the harbour, this time the fight actually could start properly and my garrison was able to come in as reinforcements and successfully defend the city.

Presumably an edge-case bug when embarked units attack a city at just the right angle. Perhaps siege defenders deployment zones should always include all city districts that are within the battle area to prevent this?