I have noticed that when you move an injured land unit to one of your cities the behaviour is as follows:
Naval injured units behaviour is correct when you select "fortify until head" in a city. It returns the unit to your control once the unit is fully healed.
- If you select "Fortify until healed" for the injured unit, when it heales completely the unit is auto-garrisoned in the city, instead of auto-selected it so you can use it. I usually move an injured unit to a city to heal it quickly, not to garrison it. I think this is a bug: "Fortify until healed" should return the unit to your control, regardless if it is in a city or in a non-city square.
- Sometimes, you don't even have the "fortify until healed" order for your unit. It garrisons automatically.
Naval injured units behaviour is correct when you select "fortify until head" in a city. It returns the unit to your control once the unit is fully healed.