I have InDesign 16.0 with a Dell Windows 10 platform. When i use the file->save option InDesign randomnly chooses the folder from all of the document folders opened during the current session for the document to be saved in, rather than the existing folder where the document to be saved originated. Since on any given day, i might have opened 10 documents in 10 different folders, i have remember EVERY document worked on to hunt for my file. It used to be that if i used file->save InDesign would automatically target the source folder from which the document opened.
Please vote this thread about this issue