The movie has up until now been listed as taking place “after the war’ in the TV series.
Following this week’s episode of Kamen Rider Build, the series’ official Toei website announced that the movie will take place between episodes 45, which aired this week, and episode 46. The movie’s story is stated to take place “after the war” in the TV series and takes place in an era where the country of Japan is on its way towards healing, with new governors put in place to lead Touto, Hokuto, and Seito.
Kamen Rider Build: Be the One will be released on August 4th in Japan.
Source: Toei’s Kamen Rider Build blog