Masayoshi Son wants investors to believe that a record loss from investments in money-losing startups WeWork and Uber Technologies Inc. is but a bump in the road. Some aren't buying it.

SoftBank Group Corp.'s shares fell as much as 4.2 percent in Tokyo trading on Thursday morning, the biggest intraday decline in about six weeks. The conglomerate recorded an operating loss of ¥704.4 billion ($6.5 billion) after writedowns in WeWork and other investments, its first such loss in 14 years and the biggest quarterly shortfall ever. The $100 billion Vision Fund, the unprecedented investment vehicle that had been producing big profits, lost ¥970.3 billion.

At a briefing in Tokyo on Wednesday, the billionaire admitted that "earnings are a mess." He allowed that overvaluing WeWork was a judgment error, but then spent the bulk of the nearly two-hour event defending his approach, highlighting the potential of his technology investments and boasting about his returns compared with traditional venture capitalists. He made it clear that he has no plans to back off a strategy that has rattled Silicon Valley and raised concerns of a bubble in startup valuations. Indeed, he said that fundraising for a second Vision Fund is on track and the fund will debut soon.