It took me some time to check before the answer, sorry for the delay.
That is more features then a bug, but when I was implementing that I have also thought that is a bug... If I remember correctly, I have even "fixed" it once (just to find that really break more then cure). Let me explain.
Key combinations use Sonar Commands. In this case "GoTo previous marker". And then the behavior you observe from Marker Action will be buggy. But if you really want to navigate markers, please use Commands instead. There are also a bit more logic in such commands, effectively during "Play": you normally want jump to previous marker in case during play you are "near" current one, since if you press "Previous" repeatedly, the Time cursor will always advance during presses and pressing will always bring you to the same marker if that is not taken into account (Sonar is smart, it does that).
Marker Action in AZ Controller was introduced for different purpose, not as the command duplicate. Let say you have buttons with markers on your surface (f.e. VS700 or AZ Controller OSC Phone preset). In that case, you normally have "Previous"/"Current"/"Next" labeled buttons. "Previous" is -1, "Current" is 0 and "Next" is +1 (from Cursor). In this case, you want "Current" button (0) return to the beginning of the current marker, not the "Previous" (-1) button! In other words, you want "Text" mode is in sync with "GoTo" mode. I hope you can understand the intention now.
Note that "back from" and "forward from" are there for State shift. Since states can not be negative, I need something to define the direction for the shift.
I repeat, I know all that is confusing (even for myself). I just have not found better way to do/expose that in the interface