Yeah, just happened in this game. Clan joined while we were doing the Nertzes quest part where Aserai, Battanians, & Vlandians are in unending war with us. Took care of Aserai, then clan joined us just before I started on Battanian area. Took Uthelheim Castle, it was awarded to them. Then took Seonon, it was awarded to them. Less than a day later he defects to Vlandians. In real life, why would any lord defect if within a week and a half he has already been awarded a Town and Castle he had no part in taking.
For the record, since the Neretzes thing started, I've lead the army that took all 5 towns and 5 castles the Aserai lost before they offered peace. So far I've taken back a castle we had lost and the castle & town noted above. So I've led the army that took 6 towns, 6 castles, reclaimed another castle, and defeated at least a 1/2 dozen other armies in the field. I've been awarded none of the holdings we took and in addition to the 2 holdings lost above, so far 1 of the Aserai towns I took has rebelled. Additionally, 2 other towns previously taken by armies I led & which were not awarded to me have rebelled. About the only thing the rest of the Southern Empire does is occasionally take back a holding that was lost and mostly retake rebelled towns so they can rebel again a little bit later.
Yeah, this is one of the better games I've had. Have gotten some real good items from tournaments and occasional drops & my family & companions are well outfitted. I think the only battle I've lost was 1 hideout (although I tried to retreat in another one, had to auto-fight it with me injured, and managed to win it). But the above scenario combined with the fact that I've pretty much led the armies that took the territories that pretty much doubled the size of our kingdom, but have only been awarded 2 towns and 2 castles, none of which are my native culture, is very frustrating. Keeping what I've been given with the -3 culture penalty, while having to make sure we don't lose everything I've gained for us has been very frustrating. Its too bad none of the Taleworlds Devs play the game. They might understand better why its frustrating.