Preserve mode appears to only preserve from when it was created.

It appears as though when I create a delta in preserve mode, if I change an option in the original theme, that option is not reflected in the delta, even if I have not overriden that option.

e.g. After I create a delta, I discover I need a new CSS file. I add it to the original theme and select it. When I go into the delta, the CSS file appears in the list but is not selected. From the description, I'd expect that anything I don't explicitly override would be inherited.

Colin