I am experiencing the same issue at least since my watch updated to Watch OS 6.0.1.
Devices: I have an iPhone XR running OS 13.0, and a watch series 3 running Watch OS 6.0.1.
Issue prior to troubleshooting: Reminders don't show on my watch (although I cannot verify that they ever did because I never checked), I can't dictate Siri to set reminders on my watch anymore (error message: "Sorry. I wasn't able to create that reminder." and reminders set on my phone do not pop up on my watch anymore (while they do pop up on my MacBook). (Other apps are working perfectly fine across my devices.)
Troubleshooting: I tried
- disabling and enabling iCloud syncing for reminders
- reinstalling reminders app on phone and watch
- restarting watch
- restarting phone
Issues after troubleshooting: Same as above.