When  a svg path is of class "x" but has also inline extra attributes, then the class "x" attributes are not applied, only the extra inline attributes are applied. This can lead for example to non-drawing issue when the color attribute is in the class: it is then not applied.

See proof test example source code here:

http://brouits.free.fr/tmp/qsvgrenderer_css_mixed.zip

 (because attachement seem broken here).

the example should draw 3 vertical lines. only two are printed. one full inline, one full class. the third line with mixed class and attributes does not print.