This is partly normal. It happened to me in early access after sparing the owl bear and long resting. Eventually, the goblins will always kill the mother owl bear and kidnap the cub.
It may be that if you send these two there, they will now always kill the mother owl bear and cub unless you specifically spare it at end of combat. It may also be that the cub was killed in battle and you didn't notice? The cub's death is the only unusual development
