Kant's test implicitly relies on the idea of some sort of deviation from the norm. If the norm is that everyone punches each other in the face, and I decide to merely slap you, that's an improvement; if everyone did that it would be a net good. But if the norm is a polite handshake, then a slap is worse. Kant's test is always relative to expectations.
Expectations are what distinguish "honest whistleblower with a love for truth" and "propaganda merchant". Leaking information hurts people. The ethical defense of it is "public interest" - that it's a net good when truth is more widely known. If you're prepared to leak information, I expect you to leak all of it. When you selectively release information, you cannot expect me to believe that public interest is really the principle you are abiding by, and any beliefs about the world derived from that information come under suspicion, as they have been engineered.
Kant objected pretty strongly to lies, and so do I - even lies by omission.
Also, you don't necessarily have to be partisan to want to release a story at time that's inconvenient to a party. It's natural for a journalist to want to break a big story, and it's also more beneficial if an important story reaches more people. If you want some important information to be shared widely, one of the best times to drop it is when it will cause a nation-wide sensation.