@cvx implemented a fix for the spoiler elements, the blurring is one-way for elements that have their own click actions (lightboxes, animated gifs, links, videos, inputs, etc.). See this commit for more details.
In the preview, spoilers are toggleable. In posts, there is a bit of a workaround where you can click in the space next to an image to reblur it. You can try this out in the animated image posted by Kris above.