Yeehaw! I found the problem with Multiplicity.
Apparently, if you have multiple network adapters installed (even virtual ones like a VPN or for VMWare/VBox), the adapter that is pinged with a magic packet may not be the correct one!
This does NOT affect the Seamless or KVM control code, because once the Secondary Computers are awake, they can be Connected To or controlled just fine, with no problems.
However, they still can't be sent a magic packet to wake them up even after you've controlled the same machine, put it to sleep, etc.
I tested this by disabling the other network adapters and then unloading Multiplicity. Once I re-ran Multiplicity with only one adapter enabled (the wired adapter), the wake-on-lan functionality of Multiplicity was restored. The Seamless command, the Menu command, and the Connect auto-wake-up all returned to proper working order.
So, I don't know why the wrong adapter is getting bound to this command but not elsewhere. Is it alphabetical (EDIT: nope) or binding to whatever adapter wakes up first or...
That's for you fine folks to figure out. I hope this helps.
EDIT: List of Adapters
Ethernet - RealTek PCIe GbE Family Controller - wired, correct, network adapter
Ethernet 2 - VirtualBox Host-Only Ethernet adapter - for VirtualBox Virtual Machines
ExpressVPN - OpenVPN TAP Adapter
vEthernet (Default Switch) - Hyper-V Virtual Ethernet Adapter <- I suspect this bad boy