Not a Bug
Minion lost notification
When your vampire eats a one of your minions who is in prison, you get "minion lost" notification, the same as you get when your minions killed in combat.
Customer support service by UserEcho
For Review in the new year to see whether this is something we can address. Though on the other hand we may keep it the same depending on our discussions as this could still be important information for the player.