A top U.S. partner at SoftBank Group Corp.'s technology fund is stepping down, after the company posted declining returns on its investments and struggled to raise capital for the next Vision Fund.

Michael Ronen, the outgoing managing partner, expressed concerns about "issues" at SoftBank in an interview with the Financial Times, which earlier reported his departure. Since joining in 2017 from Goldman Sachs Group Inc., Ronen led a series of investments, most notably a $2.25 billion deal for General Motors Co.'s Cruise self-driving unit. He's at least the second managing partner to leave in the last couple of months.

The $100 billion (¥10.9 trillion) Vision Fund has reshaped the tech startup market with a flood of investments, but several of its bets on unprofitable companies quickly went south. SoftBank bid up the valuation of WeWork parent company The We Co. to $47 billion before a failed attempt at an initial public offering sent the value plummeting and forced the conglomerate to bail out the coworking startup. Another investment, Uber Technologies Inc., is trading below its IPO price and faces mounting pressure from governments.

The corporate culture inside the Vision Fund can be punishing. A Bloomberg Businessweek story in December reported on allegations of harassment and recklessness. Another aspect of the culture pits technologists and financial industry veterans against one another. SoftBank's billionaire founder, Masayoshi Son, once derided an employee by saying, "You're too much like a banker!"

In 2017, when Ronen was still a banker at Goldman Sachs, he pitched Son on ideas for investments in autonomous driving. Instead, Son pitched the former Israeli air force intelligence analyst on joining the Vision Fund.

Praveen Akkiraju, who joined the Vision Fund as managing partner in 2018 from a software company, stepped down in December. Ronen's exit will leave four managing partners in the U.S.