“Full Disk Access” on macOS doesn’t disable sandboxing — it only lets non-sandboxed apps read from protected system folders like Mail, Messages, or Safari data.

Since your app is sandboxed, the sandbox still enforces its own file restrictions regardless of Full Disk Access. Even with FDA enabled, a sandboxed app can only access files the user explicitly grants via an open/save panel, a security-scoped bookmark, or a declared entitlement.

Fix: To truly access arbitrary paths, you must disable sandboxing for that build, or keep using your existing bookmark-grant system for a sandboxed app.