One of my workstations is now in the state I mentioned above: the ptp provider has bound to the "Loopback Pseudo-Interface 1" instead of "Ethernet". The GMC is on the network connected to "Ethernet" so nothing works. Still trying to figure out a way around this - adjusting InterfaceMetric, in this case, has not worked.
C:\WINDOWS\system32>netsh interface ip show join
Interface 1: Loopback Pseudo-Interface 1
Scope References Last Address
---------- ---------- ---- ---------------------------------
0 2 Yes 224.0.1.129
0 3 Yes 239.255.255.250
Interface 6: Ethernet
Scope References Last Address
---------- ---------- ---- ---------------------------------
0 0 Yes 224.0.0.1
0 2 Yes 224.0.0.251
0 1 Yes 224.0.0.252
0 3 Yes 239.255.255.250
Interface 4: Mellanox1
Scope References Last Address
---------- ---------- ---- ---------------------------------
0 0 Yes 224.0.0.1
Interface 25: Mellanox2
Scope References Last Address
---------- ---------- ---- ---------------------------------
0 0 Yes 224.0.0.1
0 2 Yes 224.0.0.251
0 1 Yes 224.0.0.252
0 3 Yes 239.255.255.250