Only one custom feature is shown in Persistent Storage UI

While doing #19222 (closed) we discovered a UI problem: the interface only show one CustomFeatures, no matter how many there are. But they all work, actually!

The problem might be in add_custom_features, where we don't clone the rows, just change the attributes for them.

I'm not putting that on FT's plate, because this doesn't sound like a very important bug.