Last October, the world's stock of negative-yielding debt had tumbled by more than half from its record high, as investors adjusted to the end of super-loose monetary policy. Now it's soaring again after the dovish pivots around the world.

The Bloomberg Barclays Global Aggregate Negative-Yielding Debt Index has increased in value by well over $3 trillion since its low five months back, to $9.3 trillion Wednesday. That's still below the all-time record of $12.2 trillion in June 2016.

As warnings over the global growth outlook abound, bond yields have been in retreat. One risk-free benchmark—10-year Treasury yields—are close to their lowest in about a year. Japan's equivalent is back in negative territory, while that on German Bunds is teetering on the brink—about 6 basis points above zero. Even in Australia, which hasn't seen a recession since the 1990s, 10-year yields are now below 2 percent.