This has been reported a number of times    , even in FE : when AI Sovereigns surrender, they usually have large quantities of Leather Greaves armor, Amulet of Haste accessories, and Horse/Warg mounts.
Assuming this is not a "Spoils of War" feature, sort of, then I suppose the reason it's not been fixed is that the mechanism by which the AI Sovereigns come to accumulate so many is unknown. After a number of tests, I believe I can report how the accumulation occurs:
Every time the player Loads a Saved Game and presses End Turn, the AI Sovereign receives an additional pair of Leather Greaves.
[ So if you want to know how many times you've reloaded, just count the number of Leather Greaves the AI Sovereign has ]
18 Leather Greaves
19 Leather Greaves (After Reload + End Turn)
20 Leather Greaves (After another Reload + End Turn)
The mechanism is the same for Horse/Warg Mounts, except you need to press End Turn more times (I believe it's under 10, but I haven't precisely verified), which is why the AI Sovereign usually has more Leather Greaves than Horse/Warg Mounts. I'm assuming Amulet of Haste duplication is happening the same way, but it might require even more Seasons before it gets duplicated as I haven't seen it taking place in my limited testing.
12 Horse/Warg Mounts
13 Horse/Warg Mounts (Approximately 10 Seasons after reload)
I'm also assuming Leatherworking (Leather Greaves), Mounted Warfare / Warg Riding (Horse/Warg Mounts) and Glyph Stones (Amulet of Haste) are required before duplication can happen.
If this can't be fixed normally, I suppose a Mod fix using new InternalNames for Leather Greaves, Amulet of Haste, and Horse/Warg Mounts as well as disabling Core files might fix it (Assuming, again, that the equipment duplication is based on InternalNames not some sort of random occurrence malfunction), but as there's so many Unit Templates using these items, it's probably better if it can be fixed in some other way.
XtraDeconstruct an LH Mod by Primal