Quick followup: I just maximized all my windows and then minimized them to the launcher, as I usually do, and this caused the problem of Unity not switching to the correct viewport when using Alt-Numeric. If I unminimize a window, then the viewport switching again seems to work.
In a nutshell, then: Using Alt-Numeric on a minimized window WILL unminimize and focus that windows but WILL NOT switch to the correct viewport, at least for me. This behavior is consistent and repeatable. I believe this to be a specific use-case example of the more general issues discussed in my previous comment.
Quick followup: I just maximized all my windows and then minimized them to the launcher, as I usually do, and this caused the problem of Unity not switching to the correct viewport when using Alt-Numeric. If I unminimize a window, then the viewport switching again seems to work.
In a nutshell, then: Using Alt-Numeric on a minimized window WILL unminimize and focus that windows but WILL NOT switch to the correct viewport, at least for me. This behavior is consistent and repeatable. I believe this to be a specific use-case example of the more general issues discussed in my previous comment.