I tested it now and probably the issue is that, when you use the "add" button to add additional credentials, even if you are using tags, the view in the pop-up is again the default host and clusters, where you see the selected tag but you cannot further expand it. If you switch again to "VMs and Tags" then you can expand the selected tag and see all the VMs that have that tag and that will then be processed by that job.
Hope it helps,
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1