Jiang Cheng is beaten to the hall by a different Jiang Cheng.
This Jiang Cheng spends the last few minutes before the ceremony is about to begin angrily fretting about the still-missing Wei Wuxian. He has no idea where Wei Wuxian is. Did he get lost? He probably got lost. He'd better have gotten lost, because if he instead did something stupid and dangerous then Jiang Cheng is going to break his legs when he gets back!
For variety, there is also being pissed off at Lan Wangji. He had apologized earlier, while they were on their way to check if the gate guard had seen Wei Wuxian yet, but had only done so to A-Jie. Apparently, the great Second Jade of Lan did not think attempted murder worth apologizing over unless it caused an injury. Lan Wangji had walked right past Jiang Cheng without acknowledging his existence.