I have had maybe 50 enemy sovereigns surrender since FE was released and they all suffered from this in one way or another. In FE it was mostly 20 to 40 leather greaves and lots of other random junk. In LH it has tended to be a lot more horses and wargs sometimes as many as 12 horses and 5 wargs with other junk leather and multiple weapons (probably from loot and some quests). I sell all the spare loot and it is often worth over 1000 gildar and that alone makes the surrender worthwile.
The multiple copies of leather greaves are obviously not all from loot, the AI must be buying them from its shop. I'm guessing but it was probably something to do with encumbrance and the AI controlling the sovereign. Perhaps it would go through a cycle like this:
1 Have gold and some empty armour slots, so improve defence
2 Can buy armour from shop. Not wearing greaves, so buy greaves and equip.
3 Check initiative and encumbrance, greaves adds defence but extra encumbrance incurs initiative penalty
4 Unequip greaves to improve initiative. Greaves end up in backpack.
5 Next turn go to 1
As long as the AI has enough gold it buys another greaves and cycles through the AI reasoning endlessly each turn forgetting that it already has a copy of greaves available. That works as an explanation for the large number of leather greaves with encumbrance affecting initiative. But it doesn't explain multiple copies of horses and wargs as once equipped they only ever increase the initiative and weight carrying ability. So something else must be involved in LH.