If user opens an OPJU file and later Appends an old OPJ file. Then click Save Project project button failed to save to the OPJU file. It wrongly saved the file with OPJ extension.

Important: Due to this issue, there is the potential to overwrite an existing pre-Origin2018 OPJ file that happens to have the same name as the OPJU file. So if Appending OPJ to an OPJU file, be sure to choose Save Project As and not simply Save Project.

Fixed in  Origin 2018 SR1.