If you open 2 different types of LibreOffice files eg. Document A.odt and Document B.ods from a folder (ie open folder, select files, then press enter), LibreOffice loses the names and opens them as Untitled 1.odt and Untitled 2.ods If you make changes to them, and click save, LibreOffice opens a 'save as' dialog. If you open 2 of the same type of document from a folder, it works correctly. If you open 2 different types from within LibreOffice it works correctly. I have only tried this for Spreadsheets and Writer documents. I'm using Windows 10 (64 bit) 1703.
*** This bug has been marked as a duplicate of bug 104862 ***