The Autoscaling function should be turn off when deallocating the VMSS
The increase/create operation of the new VM instance depends on the average CPU within 5 mins by default. When the CPU is higher than the threshold, the new VM instance will be created even when deallocating the VMSS. However, the CPU of the new-created instance will be high so that the average CPU is still over the threshould and a new instance will be created again and again until reaching the maximum number. This behavior is buggy.
The Autoscaling function should be turn off when deallocating the VMSS.