>>/19098/
 >>/19107/
I also did some tests. On a board I own I can spoil images just fine, OP images ditto - takes a reload of the page to change the thumbnail to the spoilered one.
I couldn't test on /dump/, I got an error, tried on /test/ where same error appeared. I assume as a global I can't spoiler images.
> Are OP images no longer be spoilerable only for /dump/ or is this happening on other boards too? 
Seems to be fine elsewhere.
> Has something changed in the site-wide page structure lately, and could that change be causing this? 
No.
> Or what could otherwise be the cause of OP images no longer spoilering by moderation?
I have no idea. Will think some more.