Bahamas runner Miller-Uibo won the star-studded gala in Eugene, Orgeon, at 49.08.
It was also a track record at next year's World Cup arena, Hayward Field.
The former had the Mexican Ana Guevara at 49.34 from 2003.
Already five runners in 50 seconds
- 400 meters is my favorite sport.
I love coming here and running and having fun while I try to figure out the secret to being able to run fast in the distance, the 27-year-old tells World Athletics.
She will now run a couple of 200 meters to sharpen her speed.
Even though it is still only April, five runners have already run for 50 seconds. The 2019 World Cup season, only one runner, Miller-Uibo, had managed it at this time.