SoftBank Group Corp. tumbled to a loss in the December quarter, weighed down by a drop in the value of the Vision Fund’s public holdings, just as Masayoshi Son campaigns to raise $500 billion for the Stargate artificial-intelligence project.

SoftBank reported a net loss of nearly $2.4 billion for its fiscal third quarter, with another quarter in the red for its once-heralded Vision Fund unit. The Vision Fund unit, famed for its investment in startups including Uber and DoorDash,