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So finally managed to reproduce this and fix it. The prerequisites were:

• Thread is moderated
• Spam cleaner is set to move threads to a forum
• Spam cleaner is then used and moves that thread

In this situation, we approve the thread when we move it (so it doesn't stay in the mod queue). The code was treating the "delete" from the old forum as a delete on a visible thread because we were updating the visibility simultaneously.