Ah, yeah it's actually to be expected because the width of the spoiler container is not as wide as the width of the post container.
There isn't really an ideal solution to this. Ultimately it's working as I'd expect.
The only real solution is to make the spoiler container full width, or not center the spoiler itself, like this:
Let me just see if this is a thing, generally or something to do with your style:
Testing a spoiler
Which looks like this in BB code:
[CENTER]Let me just see if this is a thing, generally or something to do with your style:[/CENTER]
[SPOILER="Test"][CENTER]Testing a spoiler
That at least makes the button appear left aligned so it's not quite so much of an issue.