My external boot volume won't startup all my Macs when it should
From what you’ve said, I agree it should work fine.

My first guess would be to check the Startup Security Utility on the iMac Pro, a feature of the iMac Pro (and other T2 Security Chip enabled Macs) that allows Admin users the ability to enable/disable Secure Boot and also the option of allowing/disallowing External Boot. It’s the External Boot configuration I think is causing your problem.

To access the Startup Security Utility, follow these steps:


Switch on the iMac Pro and as soon as the Apple logo appears, press and hold Command (⌘)+R 
The iMac Pro will boot into the macOS Utilities window
Go to Utilities > Startup Security Utility from the menu bar
When prompted, click Enter macOS Password to choose an administrator account and enter its password
The Startup Security Window will appear and the bottom section provides two options in relation to External Boot. You will need to select the Allow booting from external media option
Exit the Startup Security Utility 
Restart your iMac Pro and try booting up from your external boot drive


NOTE: The above steps require you to enter an Administrator password. If your workplace has not set you up with an Admin account you will need to ask them to follow the above steps.

Further reading:


Apple: About Startup Security Utility
Apple: About Secure Boot
Apple: Mac computers that have the Apple T2 Security Chip