Markdown tables do not work in spoilers (they do in blockquotes) with the standard syntax. In my mind this should be supported, so I think it's a [bug] rather than a [feature request] :). I tried to circumvent that via writing the table in HTML syntax, which actually works in the preview when editing:
>! <table class="s-table"><thead><tr><th style="text-align: center;">Header 1</th><th style="text-align: center;">Header 2</th></tr></thead><tbody><tr><td style="text-align: center;">Foo</td><td style="text-align: center;">Bar</td></tr></tbody></table>
But when you actually post it, it looks like this:
(Spoiler with HTML syntax)
Header 1Header 2FooBar
In blockquotes, they work with standard markdown:
Header 1 Header 2 Foo Bar