When a new Explosion is instantiated, it often seems to default Visible to false. Only in one case in my code it somehow defaults to true. I suppose it should default to true for backwards compatibility.
No clue on why it sometimes does default to true 
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Reproduction:
Create a new place in studio, insert a new Part, run the simulation, run Instance.new("Explosion", game.Workspace.Part); through e.g. the command bar. No explosion is visible and when you view the Explosion instance in the Part instance, the Visible property is set to false. Setting Visible to true will make the explosion appear as expected.
OS: Windows 7