This is caused by issues that can arise by mixing markdown syntax within other plugin based markdown. Nested parsing tends to get a little glitchy.
Due to the order in which the markdown is processed and the plugins pick up their own syntax not being universal, your best bet for mixing formatting with more feature based markdown (like details and polls) would be to use HTML inside the tags:
<strong>Bold text</strong>
Something hidden
Source: [details=<strong>Bold text</strong>]Something hidden[/details]
As for previews, they’re not always faithful to the end post as I found while testing which attributes and tags are kept. For example, markdown tends to break when you start using it inside HTML tables.
Just a heads up: @moderators Doesn’t work at the moment.