Speaking outside my circle of competence. But I don't think it's that direct. When a topic is put under an investigative lens, it's assigned investigative journalists.
These aren't folks who write fluff. They don't resonate with baseless optimism. They're men and women of hard facts. So they sniff, sniff, sniff until they find something juicy. Then they write about it. Everything they're writing is true. (It may also flatter. Underdog stories, for example.) But when the beer fridge was swapped with a gin cabinet, the journalism got noticeably darker. No conscious filtering needed.
Now that I think about it, this may also be happening in political journalism. Positive stories are dismissed as shilling. Writers write to be read. So they stop writing those, and instead write darkly.