I double-checked and it was still tagged java and still open, with no answers at all. Why did it vanish off the list?
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This doesn't appear to be documented clearly anywhere, but the Unanswered lists for individual tags filter out questions with negative score. (The main Unanswered page does not, however, which suggests this may be a bug)
Correction: the main Unanswered page appears to filter them, but have the list in a cache so a recently-downvoted answer may still appear in the list for a while)