You can run transient analysis to check the start-up cell, most concenstration should be focus on this cell's current and its voltage, you can try to broke the startup point and re-simuate the bandgap, I believe you will find an occult difference between these cases.
thanx.
Will the transient simulation show the real chip behavior of bandgap including start-up circuit? I mean if the model was correctly characterized.
What will be other factor to let bangdap fail when wafer is out if WAT data is OK?