[Galaxy S8/S8+] Screen image is blurred after focusing on object while video recording?

If you touch to screen while recording video, your device recognizes that you want to focus on the subject
and accordingly device will keep the distance to subject.

This is a normal operation concept of Galaxy S8/S8+ Camera.

To change the focus while recording a video, tap where you want to focus.

To use auto focus mode, tap      icon on screen.