As I said in my original post, yes, I uploaded such a save. I always report first, upload second, so I can give the URL of the bug report in the Save file upload. You should have had the save last July 18.
BUT...
I found a Shibl Zumr siege save that apparently led to the cited problem. So I let it run and even with the current version modules, the problem recurs! So I've made a new save at the point it can be tested.
Do this:
Load save. I've now lost 4 parties and my Cohesion is -3.38. An enemy army is creaming the first one that left me, but 3 others are unengaged and nearby.
Without unpausing, exit the siege so you can re-enlist parties.
Open Army Management. You have 6 parties remaining and a Cohesion of -3.38.
Sort by distance. You will have parties by Furnhard, Ospir, and Ingunde unengaged nearby. Re-enlist them. Notice on the Army Management screen it now reports 40 for Cohesion. As before, if you hover the pointer there it will report -3.38.
Click Done. Now you should have 9 parties at Cohesion 40. (You don't - Cohesion is still negative)
If you unpause now, the nearby parties will head for you and may actually join you, but you're still at Cohesion -3.38 and parties continue to leave you. Your supposed Cohesion of 40 was never registered by the game.
I have already uploaded my fresh file, called Shibl Zumr Breakdown.