I think I found a defect.

AFter defeating all enemy planets, and placing remote sensors in each system, I can verify that the last enemy in play had only two mining asteriods left. By 'mining asteroids', I mean asteroids in systems that are not colonizable, but in which one may capture an object having a crystal or metal resource.

I am unable to attack these, capture them, or use my scout frigates to deploy timed explosives on them.

I think I've won, but the enemy is still showing active due to these two asteroids.