Can't get assistive access - error using system events in AppleScript

We have a sandboxed Final Cut Pro (FCP) workflow extension that needs to control FCP to export the current video project. When executing an AppleScript, we encounter the error: "System Events got an error: FCPExtension (Final Cut Pro) is not allowed assistive access." This occurs despite the container app having been granted automation and accessibility permissions by the user. What could be missing from the project to ensure the script runs without issues?

AppleScript:

shareDestination("Destination")
    on shareDestination(_dest)
      tell application "Final Cut Pro"
        activate
      end tell
      tell application "System Events"
        set frontmost of process "Final Cut Pro" to true
        tell process "Final Cut Pro"
          perform action "AXRaise" of (first window whose name contains "Final Cut Pro")
          click menu bar 1
          tell menu bar 1
            tell item 3 of menu bar items
              tell menu 1
                tell menu item 12
                  tell menu 1
                    set menuItems to menu items whose title is (_dest & "…")
                    if length of menuItems > 0 then
                      set targetMenuItem to item 1 of menuItems
                      if enabled of targetMenuItem then
                        try
                          click targetMenuItem
                        on error errMsg number errNum
                          error errMsg
                        end try
                      else
                        error "Share destination is not enabled." & return & "Please try selecting an item in Final Cut Pro."
                      end if
                    else
                      error "Share destination not found."
                    end if
                  end tell
                end tell
              end tell
            end tell
          end tell
        end tell
      end tell
    end shareDestination

As soon as the script reaches set frontmost of process "Final Cut Pro" to true it launches the error.