libreoffice --convert-to xls writes OpenDocument format instead

I use libreoffice to convert .xlsb (new Excel binary format) files to .xls (old Excel format) in order to parse them with other code that expects .xls. The following command line usually works:

libreoffice --headless --invisible --convert-to xls f.xlsb

This writes an output file f.xls. I have used this successfully for some time (driven from code, so it is not that I have just forgotten the right command line).

But now I find that the command appears to work, but the file f.xls is not Excel format at all, but an OpenDocument spreadsheet (LibreOffice's native format). Something is causing the program to ignore the specified destination format.

This is on Fedora 20 using the Fedora-supplied libreoffice package. What could have changed to break it?