I've reuploaded the custom spoiler image just in case (images stored in the database getting broken wasn't new), but the bug is still present on /dump/
Currently on /dump/:
-posts inside threads can be spoilered perfectly fine
example of regular post getting spoilered by mod-action: >>>/dump/1188 -op images instead remain unspoilered after action
example of op image not getting spoilered by mod-action: >>>/dump/1184 <- pictured, the site gives the notice that the image has been spoilered, but nothing really happens, and the image remains unspoilered
I've noticed the different layouts of your screenshots, makes me think this may be a browser compatibility issue, but it's strange, the spoilering of op images did work before, so it shouldn't be a browser compatibility issue, since nothing changed on my side in that regard.
I'll test the spoilering with a different browser just to be sure