An Impressionist masterpiece by Austrian painter Gustav Klimt was dubbed the “star” of a Sotheby’s auction when it sold for more than $53 million this week. But nagging questions remain about the Nazi-era ownership of “Island in the Attersee,” a waterscape completed between 1901 and 1902 that was bought by a Japanese private collector after a seven-minute bidding war Tuesday. Austrian newspaper Der Standard, along with OpenArt Data, a Paris-based social media platform that analyzes art world provenance issues, have both questioned the expert analysis that the auction house used to trace the sale of the painting to Vienna in...