Usually, flagging as spam will generate automatic downvotes.
This morning on Super User I noticed this wasn't happening.
This is one that was deleted quite quickly, but still only shows -2, one of which was mine (added manually), as you can see from the blue arrow.
This doesn't seem to be a duplicate of Post not getting automatic downvote on abuse flag which appeard to have been sock-puppetry or similar.
I managed to grab its original URL from the front page, which I'd not yet refreshed - https://superuser.com/questions/1531904/ if that might be helpful.