Workfront Search API Issue: Documents from deleted folders not appearing in root queries

I'm using the Workfront Search API to retrieve documents for a project. When I delete a folder (keeping its contents), the documents correctly appear in the root folder in Workfront's UI. However, my API document search queries aren't returning these documents.

What I tried:
- folders:ID_Mod: "isblank" - Only returns documents that were never in a folder, but misses documents from
deleted folders that currently live in the root.

- Various search modifiers with "isblank", "isnull", filtering by folderID, name

To me it seems there is some remnant of these documents having been under a folder previously, and that's causing an issue here? Or I could just be missing something.

 Has anyone found a server-side filter combination that works for this use case, or is client-side filtering
the standard approach?