[Fyron]

Are the PPPs targeting specific planets, or random planets? It is possible, though I am not sure how likely, that SE4 decided to target them all against the same planet, so all projects after the one that succeeded would naturally fail, as you already own the planet... I would hope that this does not happen, but it is a possible explanation other than the unlucky one. Or maybe some had targeted planets that you conquered with troops on that turn? There is no other factor besides the 50% rule that will cause a completed PPP to fail (other than the usual CI issue).