No, he's not moving fast. I dropped my army except for path finder heros in troop quarter to test this, he was moving with like 7.something speed, but disappered once I almost caught up. And I searched nearby castles. No trace, he was just gone. This has not happened with other factions, I haven't had issues making peace with remaining Norvos lords (even if they declared war on me again when I raided their temple), they even slowly recovered troops without having centres left.
So I'm pretty sure this is bug :C
So I'm pretty sure this is bug :C