If you upgraded your iPhone or iPad to iOS 13.1/iPadOS 13.1, there is an incompatible version of Reminders in those upgrades. When you first run the iOS Reminders application, there is a very clear notice that warns that choosing to run it will result in compatibility issues with macOS Reminders prior to Catalina. You had the clear option to do nothing and keep that compatibility. You must have ignored that advice.
You may still see your Reminders via Reminders for iCloud.
See this link about the Reminders issue.