@cfreeze2014it's not PAN-OS problem by default vm-series is limited to a total of 25 interface and 1 management interface , the fallback maintenance problem occur when you added a vNIC with wrong driver type for example
if you use vmware you need to modify type of interface added from e1000 to vmxnet3 in order to do that you need to modify file : *.vmx in vm store
for KVM : same problem occure if you dont use vNIC virtual driver : virtIO