Supersonic vehicles produce Mach cones behind them, which observers hear as sonic booms as they pass by. So if, for example, a jet goes supersonic as it heads toward you, but decelerates to subsonic before it passes you, the Mach cone will never pass your position, and you'll never hear the boom.
So how then was the sonic boom(s) from the SpaceX Falcon Heavy side boosters audible from the ground at all? Shouldn't the Mach cones have been directed behind the side boosters, up into the sky, meaning they'd never pass any point on the ground?
Note: There is a question that uses the Falcon Heavy boosters to ask about sonic booms in general, where even though the boosters are mentioned in the question it's actually just asking about general principles of sonic booms rather than about Mach cones in the Falcon Heavy specifically here.