I'm not 100% sure it's a bug, but if it isn't there should be more informative tooltips at least:
As Littlefinger, I picked a plot to murder my wife, Lady Morya of the Vale. Just by myself I had about +135% plot power, so I went with the "Hire Assassin" option that gives 75% succes / 25% failure. It failed, but after that, even after waiting a few years, I was stuck with (a) being unable to change my ambition since "Plots in motion cannot be cancelled!", and (b) having no new plot-related decisions at all. Last time I had an assassination plot fail I could at least retry a couple of years later.
(Incidentally, although this is more for the game balance talk, there ought to be some benefit to having something ridiculous like +400% plot power, instead of being always stuck at the 75% chance as best.)