When a member uploaded images with a spoiler tag applied, the images were fully visible. When I go to edit the post, the preview shows the images as correctly blurred, but the images keep being openly visible.
Seems you use two images
[spoiler]

[/spoiler]
So the problem is reproducible, if two images are used.
Nice catch @moin
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For more than one image on separate lines I think you either have to wrap them separately for individual blurring, or have the wraps on their own lines like this for them both to be covered:
[spoiler]


[/spoiler]
Thank you all for the clarification!

