Unless that's a new change, it never worked that way before.
I have the opposite occur. If there are enemy military forces defending a system, they will NEVER retreat, even if they are defending just an outpost. This applies only to combat hulls, not unarmed ships.
The reason the computer design does not use heavy weapons is because of the low damage output from them compared to the standard version. About the only one it will use is hellbore.
An outpost is a space station deployable to worlds you do not control. It does not add an orbital slot, so either deploy it to the uninhabited world and colonize afterward, or just build the orbital you want after world is colonized.