Chickering suggests that bonds were seen as a patriotic sacrifice and they were nominally honoured but their value reduced by inflation:
A war bond with a face value of 1,000 marks when purchased in the
summer of 1914 still carried a face value of 1,000 marks in the summer
of 1918, when its value, adjusted to current prices, stood close to
300 marks.
They could do this because they issued bonds in their own currency to a largely domestic audience and transitioned off the gold standard to a fiat currency during the war, as explained in this article.