Yesterday I tried replacing my ZW with a 185 brick because I was pushing the ZW to the max on one island of the layout. It had mainline plus a large yard. But after rewiring with the brick, I had the same problem.
Today I spent two hours on the cold concrete floor separating the mainline from the yard today so I could use different power supplies for each. When I connected just the mainline, things were good. But after reconnecting the yard, I’m getting a power bleed between the main and the yard. I can’t track it down. It could be power flowing through a Ross switch that I forgot I soldered so power is flowing through it. It could be some feeder wire somewhere I’m missing because I never intended to separate the main and yard so I didn’t care where the power came from.
I’m sure there’s an easy way to find the bleed but darn it, I don’t know what that is. Back to poking around tomorrow. I’m sure I many of us have been in this situation.