This applies to the latest build:

KDE Frameworks 5.27.0
Qt 5.7.0 (built against 5.7.0)
The xcb windowing system

Opening a document from dolphin via either smb:// or sftp:// in e.g. libreoffice, there is no way to 'save as' back to the same network share. The problem appears to be with the temporary file created in $HOME/.cache/kioexec/krun/. The 'save as' dialog does not relate to the original location of the file, but instead to the temporary location created in .cache/kioexec/krun.

This effectively prevents any meaningful use of network stored documents. I have confirmed this behavior with both smb:// and sftp:// kio-slaves.

This may be related to, but is not a duplicate of, https://bugs.kde.org/show_bug.cgi?id=253547  (this involves the simple access of and saving of documents, not streaming contents)

The sftp:// kio will work correctly from some kde applications (e.g. kwrite), but does not work at all from others like spectacle and does not work from libreoffice (even with KDE integration)

In KDE file 'save/save as' dialog and kio should work regardless of application. At present, there is no way to 'save as' on a file opened from dolphin in libreoffice (which defeats the purpose of opening the network file to begin with unless you want to overwrite the original). The spectacle 'save as' is the subject of an existing bug, but seems related here. There is no ability to 'save as' to any network location, while in ksnapshot that was a trivial task with the sftp-kio. 

The ability to 'save as' using kio seems like hit-or-miss functionality in the current KDE. Without it, you are back to the decades old scheme of saving the new file locally, then scp or sftp the new file back to the proper location on the original server. That's doable and better than a floppy drive, but not really what is usable today. The expected behavior is to be able to save or 'save as' to the desired local or remote location from any KDE dialog.

To reproduce, use dolphin to navigate to any remote location holding a libreoffice document and open the document. Try and 'save as' to the original location. Instead of having a path to the original server, you are presented with $HOME/.cache/kioexec/krun/. Similarly, take a snapshot with spectacle and try to 'save as' to a remote location.