I had this happen in a gigantic game while playing the Yor:
I had five large ships in a fleet with a single terror star, would move adjacent to a star, move the five ships outside the blast radius, nova, and move them back.
What I found was: If I used the fleet management screen to remove the terror star from the fleet, my fleet would have a movement of 1 pc. If I clicked disband and reformed the escorts into a new fleet, they'd have their normal movement of 12 pc. I was able to repeat this. I am certain I didn't accidentally remove an escort instead of the terror star.
Has anyone else seen this?