Ok. I think I know what's going on.
We're using a plugin called support forum that looks like it hasn't been updated in quite a while. My guess is that it keeps a hard reference to the
tid, even if it's moved to another category.
It's only 168 lines of source, but I don't know Node.js or NodeBB architecture enough to make heads or tails of it. Maybe I should put a bounty on it? Not sure how well those kinds of offers are received, though.