Can you try to reproduce this issue on 1.5.6? I've been informed that it is fixed in 1.5.6.
Ok Im here to report my findings in 1.5.6.
To begin with, some things did change. Not sure that it was for the best. Now armies have more food and more stable cohesion (which is a good thing since it allows them to siege for longer periods), BUT they are sieging the settlements with an army of the same quantities as the garrison force. What does this lead to? It leads to same thing I described previously. They come, they siege you for 10 days trying to break your walls with siege weapons, they eventually run out of food and then just bugger off. It happened overall like 6-7 times.
The only way for the AI to proceed to Assault was when an army that was 2.5 times my garrison force besieged, broken 1 or 2 of my walls with trebuchets and ONLY then it decided it is good enough to actually attack. I believe
(not proven by testing yet!) that if I max out my siege and normal workshops in the city and I would be able to destroy enemy siege engines for 10 days before they break my walls, enemy army no matter what size it is will just turn around and leave. I have an example of an army 2x my size sieging for 10 days, failing to break the walls and leave.
So no, I believe the issue is still there and it may have just became even more annoying to reproduce since I have to wait for 10 days in a settlement for enemy army to leave so another one can attack.
Screenshots attached: