SoftBank Group Corp.’s already controversial stock and options trading program is drawing fresh fire from investors because of CEO Masayoshi Son’s personal stake in the trading.

Son faced a barrage of questions about the transactions on a late-evening call with analysts and fund managers after an earnings announcement Monday, according to people who took part in the briefing. SoftBank has poured about $20 billion into tech stocks and derivatives through a unit in which the billionaire personally holds a one-third stake.

Several people on the call pointed to the structure as a corporate governance concern, the people said, asking to remain unnamed because the details of the discussion aren’t public. Son denied there is a conflict of interest and described it as remuneration for his investment expertise. Other fund managers charge fees, he said, according to one of the people. Son added that SoftBank’s board cleared the structure in a vote from which he recused himself, the person said.