Thank you for taking the time to visit the Community to report this issue, although I apologize that you have not received a response.
I would recommend running the Validate & Repair tool on the data file from the File menu > File Operations > Validate & Repair option.
In the Validate File window that opens, click the top box to "validate file" and then click OK.
When the validation completes, a data log will open in Notepad, please let us know if any errors or issues are found/repaired, and when ready close and re-open Quicken.
Please let us know what you find!
-Quicken Tyka
To confirm, please navigate to Edit > Preferences > Mobile & Web, there will be an on-off toggle. If enabled, can you please disable sync and see if this prevents the reminders from reappearing?
Please let me know how it goes.
Thank you for your response and the additional details, although I apologize that this trouble continues to persist.
At this time I would recommend contacting support directly for advanced troubleshooting steps.
https://www.quicken.com/support#contact-support
Quicken Care has the tools to review and research this issue further.
Thank you,-Quicken Tyka