That really doesn't make much sense. The reason the tenders stay quiet when you have them connected to the locomotive is they are seeing the serial data. If they're seeing the serial data, then the commands should make it through to wake them up. The fact that it affects multiple units is even more baffling, I'm not sure what to make of that.
Since it's almost certain to be track, signal, or some oddball interference, I'd start eliminating causes.