Yes, it seems that something weird happened there.

I had a save from end of the previous turn and reloaded it several times to always see the same thing happen.

Out of curiosity I started another campaign with Hojo and exactly the same settings. I followed exactly the same opening moves. The problem did not repeat. Uesigi attacked Mutsu from Dewa and retreated without a fight but his men remained in the castle.

The problem has also not, as yet, reoccured in the campaign where it took place. That's a promissary note as I am only about halfway through it.

I've kept the savegame as a point of reference in case it happens again in the future.