Here's the mongoid.yml file that gets loaded into my sidekiq process:
To reproduce the issue, I queue up 100 sidekiq jobs that read & write from my_db. Then I start a sidekiq server. After sidekiq boots up, there's a >50% chance that I start seeing "not authorized" errors. These errors will persist until I reboot the sidekiq server.
Switching to "auth_mech: :mongodb_cr" seems to fix the issue.