China’s economy is slowing so much it even tripped up Apple, while the clock ticks on the 90-day cease-fire in the U.S. trade feud. Less noticed amid the headlines is a major shift in the Chinese economy that is quietly beginning to upend the global financial system.

China, long the world’s heavyweight saver and a huge buyer of foreign assets like Treasurys, is now a big spender—a trend the current cyclical growth dip is unlikely to permanently derail. In early 2018, China got more of its growth from consumption than the U.S.,...