Weird Campy 11speed rear derailleur cage issue

[IJWS]

This might be too strange to expect anyone to have experience with but here goes:

The chain on the upper pulley of my Record 11 rear derailleur rubs. Only while coasting or backpedaling in a stand.

I started hearing this weird rub a couple months ago. Sometimes when I was coasting downhill, there would be a rubbing noise ONLY when I was coasting. If I pedaled it would go away. I could not figure it out.

Yesterday when I was walking my bike down the hall, the noise was VERY apparent. When I looked closely and started fussing with it, I saw that a portion of the chain was off-axis from the top pulley of the rear derailleur cage--maybe something like 20% of the chain was shifting over towards the outside (away from the bike) of the cage.

It would make that noise, then the noise would go away. After yesterday's revelation, it has been a telltale heart situation--it's all I can hear now and it stresses me out. I tried to ride today but I would hear the rubbing noise every other minute. I think something is wearing and making the issue worse.

Has anyone ever experienced something like this? I have a feeling that it is an easy fix. Something like "new cage assembly" "new chain" and you are done; but I want to know what I am fixing and maybe if I can just repair the issue instead of throwing parts at it.

Any comments/advice/experience is appreciated.

Thank you Paceline!