Uber's plunging stock price has left its biggest stakeholder in the red.

SoftBank, which poured about $7.6 billion into Uber in early 2018 after the ride-hailing company suffered a series of bruising gaffes, has seen the value of its stake dwindle by the day. Uber shares fell almost 6% on Tuesday, dropping for the eighth time in nine trading sessions as they headed for a record low close.

The stock has lost close to one-third of its value since its May IPO.

Uber is at the centerpiece of SoftBank CEO Masayoshi Son's plan to build up a portfolio of next-generation transportation companies that, in his view, are all moving towards a future of artificial intelligence and autonomy. While gyrations in Uber's share price are unlikely to alter Son's thesis, the company's mounting losses and unclear path towards financial stability are proving unappealing to public market investors.

At under $30.70, where Uber closed on Tuesday, the stock is below the lowest point at which SoftBank invested. The Japanese conglomerate spent about $6.6 billion to buy just over 200 million shares from existing investors, including former CEO Travis Kalanick, at around $32.87 a piece. It provided another $1.05 billion in fresh capital to Uber, buying 21.45 million shares at about $48.77 each.

In total, SoftBank invested $7.65 billion. That stake, when factoring in the $245.3 million in shares SoftBank sold in the IPO, is currently worth less than $7 billion.

SoftBank bundled its losses from Uber along with other investments in its latest financial report for the three months ended June 30. The company recorded unrealized losses of 195.3 million yen (about $1.84 billion) for "the decrease in the fair values of investments in Uber and others."

The $100 billion Vision Fund held 81 investments at the time of that report with costs totaling $66.3 billion and a current fair value of $82.2 billion, SoftBank said. It highlighted Slack, DoorDash and India's OYO as three investments that have increased in value. SoftBank also has a massive stake in office-sharing company WeWork, which is set to go public soon and may face a similar challenge with public investors because of its high cash burn.