Ordinarily, Cmd+` switches between windows, but when you're Cmd+Tabbing (when the switcher is active), Cmd+` is essentially the same as Cmd+Shift+Tab (i.e., it selects the previous application). It's actually much more convenient to press Cmd+`, if you get used to it :)
That's possibly a valid workaround (can't test - my K811 keyboard is at the office) which on my international keyboard is not nearly as convenient as just depressing shift with my pinky, but it does not change the fact that the K811 keyboard is not able to do what the standard Macbook keyboard does. And the fact that Logitech is aware of the obvious bug but is not interested in fixing it.
