The way you describe it, it surely doesn't seem likely that the intended behaviour is that it should be so easy to accomplish, so report it in the bug forum, preferably with a link to a game save that'll allow reconstruction of the situation. E.g. "I've started plotting and invited duke X, then saved, when you reload the attached save, note that I've got 102% chance of success, so I press OK and become king; This is wrong since a) I am weak, b) he just fought to become king, c) there's no reason to give in to me, d) it sucks that it is so easy".Thing is - I dont think it IS a bug .. I think it's more an inherint flaw in the plot system as a whole. No matter the game (and I've played loads the last 6 weeks) - Plots are just way way way way too easy (in my experience) to achieve things with. I've not had a single one fail out of the 100s I've instigated once I've had a backer - and I've only ever waited a short amount of time to get a backer.
So please do report it; It really does help when such aberrant behaviour is reported in the proper subforum (and preferably with steps to reproduce given, if possible). Now, it'll also get noticed here due to the number of betas and moderators present, but it is so much simpler when people report buggy behaviour or behaviour that seems so out of whack with the expected in the bug forum.