Sam you mentioned that you also had a DCS set. Assume you have the legacy ground tied to the DCS common post.
I had a similar problem with the Legacy set. The track is powered from a ZW feeding through the variable channel of the DCS set and the Legacy connected to the dCS common terminal. Legacy and TMCC engines started immediately in conventional mode.
At first I thought it was a poor connection of the legacy set so tightened it a bit and things worked ok. Several days later, same thing but this time sounds in the Legacy and TMCC tenders like a buzzing. Thought it might be arcing somewhere. About a month later the DCS set failed , blowing a fuse on fixed one channel which did not have track power flowing through it. The DCS power supply I think is on fixed one internally.
I removed the DCS set from the track power and the Legacy set with the ZW operated properly. The DCS set has since been replaced with anew unit. The one that failed was a Rev L TIU, latest design.
I don't know if this experience was similar to yours but thought I would mention for your info.