This has happened to me a couple of times.  I select a Caravel or Corsair as a Prize to send offensive troops against - after the troops are underway, the prize changes from Caravel/Corsair to Safe Haven and all my offensive troops get wiped out.  This obviously is very frustrating.  The first time it happened I wrote it off as "you picked the wrong prize".   But then It happened AGAIN and I am certain I chose a RED Prize and NOT a GREEN prize.   Lost almost 3000 units of offense - juggers and all