There's some evidence that at least some of his work on those elements was also initially falsified - it seems like he was faking data to get the results he expected so he could claim the discovery ahead of any other team, and then conducting the experiments properly later.

It's just that the first few times, his hypothesis about the qualities of the new elements proved correct enough that his 'results' could be duplicated and so nobody looked closely at his work, while with the later elements, he was way off and nobody else could get results even close to what he was claiming.