Investors who bought Aramco shares after the Saudi oil giant’s highly anticipated IPO last month haven’t benefited from the outbreak of conflict in the company’s Mideast backyard.

Investors who bought Aramco shares after the Saudi oil giant’s highly anticipated initial public offering last month haven’t benefited from the outbreak of conflict in the company’s Mideast backyard.

Shares of the company—officially called Saudi Arabian Oil Co.—have fallen 10% from their peak on Dec. 16, a few days after they began trading on Saudi Arabia’s Tadawul exchange.

Over...