Manual Focus & Tracking Auto Focused Objects of Galaxy S8/S8+ Camera

If a subject is focused after touching the screen, the focus will maintain for 3 seconds showing the manual brightness control level. 

However, if your camera moves to another subject before the 3 seconds, the camera will still focus on the distant subject that you selected manually. 

This is normal concept and accordingly, your S8/S8+ camera keep the distant subject that you wanted to focus.

Tracking Auto Focus

If you want to turn on automatic focus and track a subject selected on the preview screen, please activate “Tracking AF” feature.

On camera > Settings > turn on "Tracking AF" feature.