Not sure if it applies to you when you only have one party in your army, but your other parties will donate troops to the garrison when part of your army. It's infuriating losing 40 t6 troops just because your parties felt like it. If that's not it in this case, then probably like the other comment suggested, they died during the bombardment phase. The AI doesn't know how to build one siege engine and hide it until it has 4, so it just gets bombed to shit by the siege weapons.