Sianai isn't acting OOC, he's trying to claim the gains of Arteras as his own, based on his earlier position of satrapal authority over the far north granted to him by the previous Redeemer.
I was going to write a story clarifying this, but this is essentially a salvo of Sianai against Arteras, who is manipulating the lesser princes into expressing his objections for him, as you saw in the subsequent diplomacy. (They think it's unfair too.)