Barbarian threw his throwing spear at someone, it went astray at the the circled brother with the gambeson. He deflects it with the shield, but it only does 1 damage instead of 26.
I would call this a mechanic feature, not a bug. The Barbarian Reaver first aimed at the brother without a shield, otherwise the chances of hitting are not used and the damage is dealt only to the shield. If the damage was not originally planned to be applied to the shield, then the spear works in the same way as throwing javelins/axes/bolas.