I'm having the exact same issue.
I created a new army with my MC's older brother's wife in charge of it, gave her only a few troops hoping she'll
recruit the rest and the first thing she does is entering my main town (Marunath) and stealing a bunch of tier 5 troops (a few khan's guards, a few banner knights, a few legionaries, a few elite cataphracts) from my own garrison. Then she went to my other town nearby (Seonon) and
left a couple of soldiers in the garrison (Seonon's garrison is much smaller than Marunath's).
So apparently player's clan members think they can manage player's fiefs as they please.
Many people, like myself, enjoy perfecting garrisons, and the ai just ruining it is really immersion breaking.
This.
Lords stealing my troops
literally made the game unplayable for me. I'm not exaggerating, I just can't play like this.
I do have a couple of minor mods (like extended encyclopedia or swadian armory) but I doubt any of them causes the issue.
reproduce the issue much easier and faster.
I don't thing it's the kind of issue that needs reproducing to find out what causes this.
Just check AI's reinforcement scripts. It kinda make sense that members of the same clan (since they have shared ownership over fiefs) can all manage those fiefs, but you really should add an exception (or a setting allowing such an exception) for player's clan, to stop clan members from touching player's garrisons.