Rumored to take place between Alien (1979) & Aliens (1986).

This is becoming a thing. Saw X is doing the same later this year (taking place after Saw and before Saw II).

according to claims by our friends at Perfect Organism Podcast. Where exactly in the Alien timeline does Alien: Romulus fall? They claim it will take place between the events of Alien and Aliens. They also claim the film will not ignore events established in Ridley Scott's Alien prequels, Prometheus and Alien: Covenant.