Some ridden animals lost when reforming caravan from enemy (resources) site

[SanTropical]

Won the battle at an enemy resources site.
When forming caravan, I had 6 horses, a prisoner, 7 pawns.
Tied to the caravan spot, at least 4 horses. I can't swear on the precise number.
Assigned a rider to each of the 6 horses, 1 pawn walking.
Sent the caravan out.

Message "Caravan lost": 4 of the 6 horses were left behind (lost).