Yeah, because it actually is
Ros- =
Rose, not
Ros- = Horse! Either
Rosa munda = elegant rose or
Rosa mundi =
Rose of the world, depending on one's opinion of
King Henry II's alleged girlfriend, said to have been terminated with extreme prejudice by
Eleanor of Aquitaine but in fact a later legend. Either way, it's Latin, not Germanic.
And, surely, one pronounces
Rosa with a long o?