You should keep Activity.Finish!
Your project works as expected when uncommenting Activity.Finish!
Code:
Sub Button1b_Click
Log("Hit Return")
Activity.finish
StartActivity(Main)
End Sub
Clicking on the button in Activity2 returns to Activity Main.
Clicking on another button in Activity Main shows the correct url in Activity2.
Tested on my Samsung Galaxy S8 with Android 9.
So what is your problem.