SoftBank shares plunged 6.1 percent on the Tokyo Stock Exchange at one point, the biggest drop among the Nikkei 225 components. This is also the biggest drop since the company released its financial results in May.

SoftBank's market capitalization has shrunk by nearly one-third since hitting a record high in March. Although the Japanese company posted its biggest-ever quarterly profit less than two months ago, investors were disappointed that it didn't make any new stock buyback announcements.

SoftBank's Vision Fund is Didi's largest investor, with a stake of more than 20 percent, which is also the single biggest investment in SoftBank's portfolio. According to media reports, SoftBank has invested about $12 billion in Didi.