In most cases if a reporter (and their entire news organization!) knew something to be true but kept saying the opposite for hours, that would be malpractice.
The protocol for news reporting around the death of the monarch is there for a reason: to make sure the news is authentic and to announce it to as many subjects as possible simultaneously. I don’t think it’s unusual for a news organisation to hold back from reporting something when there is a good reason. For example, troop movements, court cases, etc.
The whole thing has a cult quality to it, and it's a farce in a way even ordinary propaganda isn't - beyond embarrassment and into cringe, perhaps.
But no, even if malpractice was a thing, this wouldn’t be it.