Agreed, unplug this from your TIU and track wiring. You appear to be getting cross feed- and given that I repair these Z4000s, that's not a fault if you understand how the transformer is built. The handle cannot internally control the other channel- but the voltmeter can read a voltage coming back in on the channel.
I just tested this and can replicate the result. A wire connecting the 2 red channel outputs (since black is all internally common) results in your exact problem.
Again, it's probably NOT the Z4000, you have shorted your 2 tracks together somewhere and are backfeeding the Z4000 channel.
I would yes unplug the TIU- but since I also repair them, again, my bet is your track and wiring on the outputs is the source of this problem, not the TIU and not the Z4000.
