The thing is, it wasn't lying. First there wasn't much evidence for the myocarditis, then it was confounded evidence. Did the kid get myocarditis from the vaccine itself, or was it from the beginning of a COVID infection aborted because the immune system was already actively reacting to spike protein at the time.
Of course, over-arching all of that, COVID causes myocarditis too, and often worse so it wasn't all that clear if mild myocarditis from the vaccine would even matter. Try explaining that to people ready to eat horse paste and unsure why people are laughing at Trump's suggestion to inject bleach.
Then there's a question of how much of the distortion came from scientists and how much from journalists (mis-)quoting them?
Now that the data is in, we can see that there is some possibility of mild myocarditis from the vaccine.
The thing about science is that as more data comes in, theories change and so actions suggested by those theories also change. In emergent situations such as the COVID pandemic, data and change can come fast.
Perhaps a sports analogy. After the first baseball game of the year, plenty of batters have an average of 1.000 for the year. Plenty have .000 for the year. That will change a LOT in the next day. By the end of the season, batting averages don't move that much in a single game.