Why the soft key of back button works as "Recent button" in Galaxy S8/S8+ device?

This may happen when the “Button layout” option in “Navigation Bar” setting is changed.

If you want to change or recover the order of soft keys,

Please tap the Setting → Display → Navigation bar → Button layout and then select the order which you want.

Pictorial representation of the above settings are as follows : 

1. Tap Setting.

2. Tap Display.

3. Tap Navigation bar.

4. Tap Button layout.

5. Select the order which you want to change.

6. Now the order has been changed.