The AI rarely builds siege equipment on the first turn. Not necessarily on the second turn either. If your settlement is well protected with quality troops and a very big wall the AI sometimes has the sense to starve you out rather than attack outright.

Also if the attacking AI stack is smaller than the number of troops in your settlement it often prefers to starve you out than attack.

Check the enemy stack to see if it's led by a family member or a common captain. Sometimes the captains are less inclined to building siege equipment than family members. Could be wrong though.

Either that or this truly is a BI feature bug...