@Maik-C ,

When you pull a tab out of the current instance/window and create a new instance/window, Notepad++ actually closes the file’s buffer from the first instance, then launches a new instance of the executable, telling the new instance to open that file from disk. For a new 1 tab, there is no underlying file for Notepad++ to re-open, so it cannot do that. (The same is true if there are unsaved changes, because Notepad++ would close the file, lose the changes, then reopen the older version of the file; it disables that action to avoid people accidentally losing data.)

If what you’re trying to accomplish is “create a new file in a new instance” anyway, you could create a Run menu entry that does that: Run > Run…: "$(NPP_FULL_FILE_PATH)" -multiInst -nosession – you can save this as New N++ Instance or similar name, and even assign a keyboard shortcut to the action, if you desire.