From the Portfolio Items tree grid, I display features, and expand the feature I'm working on.
I then open a story which displays in the side pane and choose the dependencies icon.
In the following screen, I "add new" (either as predecessor or successor) and "choose" an existing user story that is already mapped to a feature.
When I refresh my Portfolio Items grid, the story which I added no longer shows under its feature. I can switch to the Plan > User Stories page, and filter for stories with no feature, and I will see the story which I added (either as predecessor or successor).
I can then remap the featureless story to its correct feature, but this is an ugly workaround.