In the movie The Wind (2018), whose baby was Emma carrying? We're shown that Lizzy kills Emma, but was this because Lizzy was going crazy or was Emma actually sleeping around with Isaac?
thisisbarry does an extensive review and discussion of The Wind at https://www.thisisbarry.com/film/the-wind-explained-2018-film-ending-explained/
thisisbarry explains why he believes that the baby was Isaac's, but the movie itself doesn't say for sure. It's a fact that the audience isn't supposed to know for sure, since this is, as thisisbarry puts it, an "unreliable narrator" story.
thisisbarry believes that there are enough clues to believe that the baby was Isaac's -- unless those clues were part of Lizzy's madness, which the viewer can't know for sure. thisisbarry's clues are the attraction he sees between the two characters, the diary that says "our baby," the desire to name the baby Isaac, and that Isaac is more concerned with the fact that Emma is dead than the fact that Lizzy is a murderer. You can check the link to see more discussion about it. There's a lot of discussion in the question-and-answer section that comes after his review. thisisbarry's review seems to be the most in-depth and collaborative discussion about this movie, at least that I can find, so you can check out what various viewers think. You can even jump into the discussion yourself if you want.
It's possible that the viewer isn't supposed to know whose baby it is. It's possible that the father of the baby isn't important to the story the writer was telling. What seems to matter in the movie is the madness of the character. Since the character can't know what's real and what isn't, it's possible that the viewer isn't supposed to know, either, to let the viewer be inside of that position of madness.