The 24-year-old was beaten in the title bout by home fighter Wanderley Pereira in Foz do Iguaçu.
Top seed in the light-heavyweight division, Veočić reached the final with wins over Italy’s Gerlando Tumminello, China’s Hao Ran Du and Canada’s Josh Ofori.
Croatia's leading boxer, a two-time European champion, Olympian and World Championship medallist, returned to top-level competition after a seven-month break following appendix surgery after the World Championships in Liverpool.