Basically, what usually happens is when selecting Open Folder in the IDM completed downloads list, the folder opens in DO showing the file selected. If there is a DO window already open, it shows the file in the same window.
In certain cases, it opens another DO window where the file is but not showing the file. Wait and another DO window opens showing the selected file.
This only appears to happen if the selected file in IDM is either an executable or mp4. I haven't got a downloaded mkv video to check that format.