We have been made aware that the 2018 Mac computer models that use the T2 security chip will not allow any external bootable devices to enumerate on these devices. Further information can be found Here that covers details about the new T2 security chips and explains the Mac models that are currently using it (2018 models).
At this time no external USBs are bootable with the T2 security chip, this directly impacts our Service Center bootable diagnostics which cannot run diagnostics on these devices. The T2 chip does allow access to some security options but even with modifying the secure boot policy to "No Security", the external device will still not be allowed to boot. At this time there is no current workaround for running Service Center on these devices but we will update this Knowledgebase once a solution becomes available.