In FontCreator 11.5.0.2427, 64-bit:
I discovered this when using the built-in “Unmapped Proportional Figures” transform script, which didn’t complete all the composites correctly because the last glyph was named “nine.pnu” instead of “nine.pnum”.
The problem happens regardless of the number of glyphs. A simple workaround is to place a comma after the last glyph name.