Thank ChrisW67 for your fast replying.
I tried as you suggested but the S8 can move forward (and skip some pages in the between), for example, to S11 not S4.

And the problem here is: there must be something difference between a real "hardware click" and the "software click" on the same button. If I use my mouse to hit the CustomButton1, it goes back to S4 flawlessly. If I click on other button, then that button fires a click to CustomButton1, the wizard still goes back to S4, but the BACK button on S4 must be clicked twice in case I want to move back to S3. Weird huh?