I agree with you analysis. If the AV was stopped completely, then it should wait until there is an opening before resuming motion, just as a parked vehicle would do.
But, if the AV was not stopped, I think the bus is now at fault for attempting to pass a moving vehicle within its own lane, which is illegal (I hope).
I don't think there's any rule of the road saying you lose or gain right-of-way depending on whether your car stops. If you park the car, yes, or if you are stopped long enough that a reasonable person would think you were parked. But just coming to stop does nothing.
In this case, I think it all boils down to vague ideas about right-of-ways when drivers casually divide lanes.
But, if the AV was not stopped, I think the bus is now at fault for attempting to pass a moving vehicle within its own lane, which is illegal (I hope).