Hi Freddy,
I'm not sure what's going on. First, no, QLab is not cycling through port numbers. There is only one port number for Art-Net (UDP port 6454), and that is all we send to. When in broadcast mode, we just send to the broadcast address of the chosen network adapter (e.g. 192.168.1.255). If there is no chosen network adapter, we send to the broadcast addresses of all the network adapters. So, if you're network adapter's broadcast address is 10.0.0.255, then that is the address QLab should be sending to.
One note - the 10.x.y.z network is a private class A network, meaning it's broadcast address is 10.255.255.255. Often people just use a class C network within it (as you seem to be doing), but this is often misconfigured, leading to some pieces configured as a class A and others a class C. Problems like yours can be caused by such network misconfiguration (e.g. your Mac thinks its network is class A, while Parallels is treating it as class C).
Other than that, I'm not sure what might be the problem. I just created a Parallels VM in bridged mode, and all of QLab's broadcast Art-Net packets seemed to get to it just fine when monitoring network traffic in it.
I hope that helps.