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.