Maybe this is a misunderstanding on my part. Is the cron guaranteed to only run once even if multiple instances are active/ready?

It's correct that deployments in Active state are the ones that are currently running. There will not be multiple active deployments at any time. Looking at your screenshot, the deployments there are in Ready state. Deployments in this state are the ones that will move to Active next, but they are not currently Active themselves. So in the screenshot, there are no Active deployments yet.

I flagged since I believe that there should not be 3 ready state deployments, that seems like a bug itself.

Thank you, @brody. I've passed on the feedback to the team that multiple deployments in ready state can be confusing.

Does multiple deployments in a ready state mean all of them will be triggered at the scheduled time? Or is this just a UI bug?

Please only answer if you are 100% certain, as I'm experiencing very strange behavior in my app after migrating to Railway:

https://help.railway.app/questions/rogue-server-need-help-asap-5f76a343

The behavior was caused by an issue on the platform on July 16, where some jobs kept running past their removal date. All jobs have been removed now. Jobs will only run at the scheduled time when using native cron jobs.