An input file such as tpc2.ps sees ps2write emit a Type 1 font (an "instance" of a multiple master font, but not a MM font itself), in which the charstrings still contain MM font operations. This causes glyph interpretation to error out.
Commit: 237f153545faec22782b64016b5f14dabb4cf6ed adds code which parses an MM font and removes the extra (blending) parameters and calls to MM OtherSubrs. This works around the problem but is not an ideal solution. The only full solution is to write a complete interpreter which converts the font into a non-MM font. See the large comment in gdevpsf1.c immediately before CheckSubrsForMM and the commit log for further details. It would be preferable to write the full interpreter using 'psf_convert_type1_to_type2' as a template, so I'm leaving this bug open but altering to an enhancement.