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Hi Martin :^)
The decoder communicates it's CVs to the programming device by presenting a load on the programming track (that's why the loco makes little jumps when you program it). It's called an "Acknowledgment" or ACK, for short. It's kind of like morse code only it's binary of course. The programmer detects the string of ACKs tapped out by the decoder and translates it into a value.
You cannot read CVs back in OPs mode programming because that happens on the full powered track. The programming track is usually a low power output that can be easily loaded down by the decoder's ACK. I made a DIY DCC system once and I never did get the ack detection working.

It sounds to me like your Zephyr's programming track is actually too powerful, rather than not powerful enough. Putting a 1k resistor across the programming track seems to have loaded it down enough for the decoder to have a noticable effect. That would be why Digitrax suggested adding a resistor.
I've never had this problem with my Zephyr but I've only ever used Digitrax brand decoders. Perhaps they have a more detectable ack signal than other brands of decoder.
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