I agree that is difficult to decipher the documentation (or even the source code), but I'm not convinced it's an issue - see my explanation/hypothesis for why the circular artifacts occur. I have yet to hear feedback from that explanation, but the behavior seems consistent to me. Also, the function is not discontinuous at 2*pi, this was induced by the grouping.
My guess is that the behavior has to do with what I understand to be the main reason for ggplot2::coord_polar - to make pie charts. I'm hoping someone here can provide some insight into that.