Foreign investors’ appetite this year for U.S. debt hasn’t grown at the same pace as the government’s borrowing needs, which some analysts worry could push bond yields higher and eventually threaten to slow economic growth.

Investors in a broad category known as “indirect bidders,” which includes both mutual funds and foreign investors, have been winning the smallest percentage of the bonds they’ve bid for since 2011, according to bidding data for recent Treasury bond auctions. The average percentage of the auctions won by...