It would appear that the function to automatically update an archive when one of its constituent files has been changed externally is actually triggered by the termination of the editor process launched by 7-Zip. This works well enough for SDI applications like NotePad but does not work well for MDI applications such as NotePad++, which continue to execute as long as they have files open. If you have a number of files open it is necessary to close the editor (and all your open files with it) in order to get 7-Zip to update the ZIP with the modified file, which is most inconvenient if you wish to continue working with these hitherto open files, some of which may have been opened by 7-Zip.
It would be much better to monitor the 7-Zip-created temporary file(s) for changes and offer to update the archive whenever any of them changes. The 'update archive' dialog could remain open until accepted or cancelled, accumulating details of temporary files saved in this way and offering to update all of them in the archive as a batch, which would be most convenient.