Have you tried temporarily swapping the same accessories currently powered by the Z, so that they're instead connected to either the B or C channels on the ZW to see what would happen to the Legacy signal on the upper loop? If it works, this would eliminate the accessories as a potential cause.
If that works as expected, then maybe see what happens when you reconnect those accessories back to the Z and then run a temporary 3 prong drop cord from one of the house outlets around either the inside or outside of the upper loop tunnel. Given the available info, this doesn't seem like it would make a difference, but could help narrow down the issue.
