If an Adversary is Revealed because they are on a Bright space (such as from a Flashlight), and then that space later becomes Dim or Dark, the Adversary does not automatically become Hidden. They must use their Disappear Action or another effect that allows them to become Hidden.

This is supported by the following rules:

• From page 8:
  "If the Adversary is on (or enters) a Bright space, immediately Reveal the Adversary by placing the Adversary figure on the main board. Adversaries typically have a Disappear Action that lets them become Hidden again."
• Also from page 8:
  "Tokens that are Revealed are never moved back to the mini-map, even if the space is no longer Bright."
• From page 28:
  "When the Adversary becomes Hidden: 1. Remove your figure from the main board and place it onto your mini-map (on the matching space it was just removed from). 2. Place a Shadow token on the space you became Hidden on."

There is no rule stating that the Adversary automatically becomes Hidden when the space is no longer Bright. The rulebook specifies that becoming Hidden requires a specific Action (such as Disappear), not just a change in lighting.

Summary:
The Adversary must use their Disappear Action or another effect to become Hidden again; they do not automatically become Hidden if the space becomes Dim or Dark.
Source: pages 8, 28.

Source: Core Rules (pp. 12, 19, 23, 28, 8)