Granted it was in a prelim and the day after Semenya won the 400 but it was the first time in 38 800 races including prelims that someone finishead of Caster in an 800.
Winnie Nanyondo of Uganda finished in front.
Video:
https://www.youtube.com/watch?v=SRA0Qx6A5yo
https://twitter.com/athleticsafrica/status/1025765621735284736
Semenya of course won the final with ease: