Rhasidat Adeleke had to settle for sixth at Sunday's Diamond League 400m meeting in Stockholm.
Drawn in lane 4, Adeleke was in the shakeup as they turned for home, but she got swallowed up at the climax in a tough field.
The American Isabella Whittaker took victory in 49.78 ahead of Norway's Henriette Jaeger (50.07) and Great Britain's Amber Anning, who ran a season's best tme of 50.17.
Adeleke crossed the line in 50.48. On Thursday, she was fourth (50.42) at the Oslo Diamond League in what was her first outing over 400m this season.
The 22-year-old is building towards the World Championships in Tokyo this September.
Earlier, Sophie O'Sullivan produced a superb performance to win the National Collegiate Athletic Association (NCAA) 1500m title in Oregon.
The 23-year-old left the rest for dust as she kicked for home to cross the line in a career-best 4:07.94.
O'Sullivan is in her final year at the University of Washington.
She's just the fifth Irishwoman to win gold at an NCAA event, following in the footsteps of her mother Sonia (3000m in 1990 and 1991), Valerie McGovern (5000m in 1989 and 1990), Mary Cullen (5000m in 2006) and Adeleke (400m in 2023 and 4x100m relay in 2022 and 2023).
