You must have a cut somewhere still. That's what stroke-miterlimit is for. See:
Ok, begining to understand
The bad thing is that Inkscape does not seem to honor srtoke-miterlimit currently, although I recall that at one point it did honor it. It's another bug that must be fixed.
I believe, that it must have, since in 0.37 I did not see that bug and just checkt with a png I created from 0.37. The tips are drawn the way I would have expected them to. That is withouth that I even set the miter.
Well, not quite. I now see that it honors _small_ values. But in a wrong way. I added a comment to that bug.
Hmm, this doesn't quite make sense to me, because if it was, the lenght of the tip would not be going toward 0.
Infinity is obviously not an option :) You can set stroke-miterlimit to a large - but finite - value. If you don't set it, the default is 4.
Hey infinity is fine :) Anyway, now I wonder if there's a interface to change the miter. If there is, I think, it should be moved to the stroke properties, if there isn't, it's needed.
David