When you drag and drop buttons from and to "Active" and "Hidden" under the Style tab of Window Manager Settings, the buttons disappear. Changes in the windows are correctly set but the dragged buttons disappear and you have to exit the window and reenter to see all buttons in "Button Layout"
That would have been a dupe of bug 13861 but this is fixed in 4.14.2 and it works fine here.
Does "xfwm4 --version" confirm that you are using the mentioned version?
xfwm4 --version 4.14.0
Sorry, I was referring to xfce4-settings which has version 4.14.2 in my system.
Right, so this is a dupe of a bug which is already fixed.
*** This bug has been marked as a duplicate of bug 13861 ***