No, running our own environment, and I found the culprit.
We were running multiple rake tasks in the same command (RAILS_ENV=production bundle exec rake db:seed_fu multisite:migrate multisite:seed_fu).
The db:seed_fu was there to work around a 10-year-old bug. When I run it separately, the problem disappears. And thanks to your comment above, I now understand what was going on.