The modern horizons 3 creature, Sorin of House Markov, failed to flip despite me gaining 3 life then losing 3 life. Maybe I’m mistaken on how certain life change interact with the card, but I think I understand how it should properly work. Here’s what happened:

My board is Sorin, a 0/1 eldrazi spawn, a 3/3, a 1/1, and a 1/2 marionette apprentice.
Precombat, I sacrificed the eldrazi spawn, triggering Marionette apprentice for one point of life gain.
I then cast Toxic Deluge paying 2 life. sorin’s extort trigger goes on the stack and is paid, causing a 3nd point of life gain.
Toxic deluge resolves, turning Sorin into a -1/2, the 3/3 into a 1/1, and kills the marionette apprentice and the 1/1, which triggers the marionette apprentice once more for a 3rd point of life gain. I then attack for 1, but post combat Sorin does not flip.

Maybe I’m misunderstanding that “paying life” blanks out life gained, but something seems wrong to me here.