imo that's more of an issue with the way the scenes are designed than anything else. They gave the bots physics so they don't phase through each other, which makes sense. But since the ai charges straight towards whoever they're locked on, only taking the shortest path (this is how it worked in warband as well btw), siege maps need to be designed in such a way that allows for greater movement. It's why in Warband mods, like Full Invasion, which can also have comparable number of ai on the field as Bannerlord currently does, scene designers make sure to limit narrow entrances to only locations that they want players to hold. In Bannerlord this needs to more the case. Entrances should be 2-3 people wide, stairs as well.
As for getting stuck on stairs, there's another issue with the scenes that can compound the issue I explained above. Quite often, the native props have sections of stairs that don't align properly (I'm thinking some of the platform prefabs specifically) and this causes bots to get stuck. Another issue that can occur with stairs (though I'm not aware of any Native scene experiencing this specific issue) is if they're too steep, the ai can't climb them.
Unfortunately it's not as simple as just adjusting the props provided, because some props, like towers, come with the stairs pre-attached. So to widen the entrances and stairs, you'd need to scale the entire tower, which could look out of place. It's still doable, but it means rearranging the surrounding walls at the same time.
Alternatively Taleworlds would need to rework the ai so that your army of 600 dudes don't all charge at a single doorway/stairway when the enemy has like 3 dudes left in their tower still fighting. This is obviously the ideal solution that should be aimed for - better battle AI, but a rework of the ai unfortunately takes a lot more time than adjusting the scenes of each siege map.
