1) Visit a workboard with a custom filter, e.g. https://phabricator.wikimedia.org/project/board/6/query/ayWdOHC5rDQc/
I. Observed: three boxes in top right state "Sort: Natural | Filter: Custom Filter | Manage Board". No details on the current filter are available. I deduce the "Filter" box will tell me what the filter is.
2) Click Filter > Custom filter
II. Observed: the current filter is described.
3) Click Filter > Advanced filter
III. Observed: a fresh form is loaded, prefilled with the defaults rather than the current state. If I want to edit my current filter and I end up here, I have to rebuild the query from scratch.
The interface wasn't obvious at all to me initially. The distinction between "Advanced filter" and "Custom filter" buttons is not hinted anywhere and I find it counterintuitive because most search engines I see tend to prefill the "advanced search" form with whatever search query you were using.