Does editing the title on old topics fix the problem?
If so, something like this might work on the rails console:
Topic.all.each do |topic|
topic.title = topic.title
topic.save!
end
(Assigning topic.title re-generates the slug)
Please please please take a backup before trying that, as I don’t know if it will have any unintended side effects.
It might also take a long time to run depending how many topics you have…