Thank you for using Apple Support Communities. It's my understanding that the keyboard in Reminders disappears when you have over 8 reminders in the list. I can definitely see how this could be concerning. I'm happy to help.
If you haven't already, try restarting your iPhone. Restarting can resolve many unexpected behaviors.
Restart your iPhone, iPad, or iPod touch
If the issue persists, follow the steps in the link below to remove and restore the Reminders app:
Delete built-in Apple apps on your iOS 11 device or Apple Watch
Please let us know if the behavior continues. Tell us if this occurs with other apps and any additional troubleshooting steps you may have tried.
Have a wonderful day!