The long-running stock bull market has left America’s wealthiest with a tax problem: too many investment gains with too few losses. With major indexes marching ever higher, and as founders and early employees of tech firms accumulate fortunes in concentrated stock positions, it’s become harder and harder for wealthy folks to identify losers that they can use to “harvest” losses to offset their capital gains.

A fast-growing strategy designed to fix this problem has become the toast of the investment industry, from the Wall Street hedge funds crafting the products to the independent advisors hawking them to clients. In the so-called long-short tax-aware strategy, an investor uses leverage to make hundreds, even thousands, of…