Previewing while editing the post displays spoilers correctly.
After submitting the post spoilers are not displayed correctly.
Experimented a little bit with this post: viewtopic.php?t=71086
Spoilers started displaying correctly after submitting only after I moved the closing tag on the last of them on the new line. After that, moving the closing tag back doesn't change how the spoilers are displayed.
Edit: I updated the linked post and cannot find any broken spoiler tags anymore
