How to reproduce:
XSSFWorkbook wb = new XSSFWorkbook();
wb.createFont(); 
//this next font gets ignored because the font above was never modified
XSSFFont f = wb.createFont(); 
f.setBold(true);


StylesTable.putFont(font) checks to see if the font is already there as part of createFont(), before any modifications on the font can occur. 

A workaround is to simply not call wb.createFont() if you don't modify the Font afterward, but this may not be viable in all cases.