Custom suffix on rendered filenames?

I have a clip named A001_C001_0111DF_001.R3D. I want to add a suffix to the filename when it's rendered, so as to differentiate between different codecs. But when I add "asdf" as a custom filename suffix in Render Settings, the result is 00586806asdf.mxf. What the heck is that? I would expect the output to be A001_C001_0111DF_001asdf.mxf.