The yield on the 10-year US Treasury crossed 5%(Opens in new window) in recent weeks, its highest level in nearly two decades. Ordinarily, that would be an uncomfortable backdrop for emerging-market borrowers. Yet several African bond markets have been moving in the opposite direction. Sovereign debt has long followed a familiar hierarchy. When global uncertainty rises, investors move towards the deepest and most liquid advanced-economy markets, while emerging-market borrowers are required to offer a greater premium. [...]