Powered block holding a button does not trigger a dispenser correctly

The dispenser in the setup below drops its items only the first time the button is pressed. Further button presses have no effect, i.e. the dispenser does no longer drop its items.

When one replaces the dispenser with a lamp, the lamp lights up as expected and continues to do so on followup button presses.

When one removes the single redstone dust everything is okay (not very surprising).
When one replaces the the single redstone dust with a block of redstone, everything is okay (i.e. the dispenser drops its items on any button press).

I have tested this on 1.8, 1.8.1-pre3, on an existing world and a new world. I have also tested this issue on a new world on 1.8 with a different pc and different account.