I also had this happen but on xbox, so different version. As a mercenary party i besieged a town and began siege camp and construction of ram, catapaut etc, the allied army came and took over the siege.
Construction continued and I was able to repeatedly build and replace catapults for cheesy engineer experience. However the army commander never launched the attack even after garrison starved out and one wall was broken.
I left before starving myself but the siege army continued to not attack, I think until they also starved out as the town wasn't taken.