this is a long time topic in VMware. vSphere assigns randomly generated mac addresses to any vm, anytime they are registered. It may seems that the mac is static only because, once the vm is created or registered, the mac stays the same until you change it forcefully from vCenter, or you de-register the vm. The only solution is to indeed set a static mac address in the vm and update the license accordingly. This static address will be replicated by Veeam then. Just be careful if the destination is inside the same vcenter, there may be some error trown out...
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1