Forgot to mention - unmodified vanilla theme.
This is a moderately clean install, but running on a slow system (I moved it off my previous VM host to replace the underlying hypervisor and it will be going back to the faster machine once that's done, so just a temporary thing, but I think it may be contributing). I installed a fresh copy of NodeBB and reinstalled my plugins and restored the redis dump on the new server. It's still on what I would consider adequate hardware for nodebb (single 3.2ghz AMD core with 1GB RAM) and fairly light usage (~10 posts per day, perhaps).
The thread in question was created after the move, though, so it wouldn't have been corrupted by the restore.
It could also be plugin related/etc.
I think a good first step to fixing it might be to fix the post count in redis? I don't really know much about how nodebb interacts with the data store, though, so if you could tell me what to look at for that, it would help.