Hello, @Bob Stone,
Welcome to the Microsoft Q&A platform!
If you use a hybrid environment, after the user reset the password, the password does not sync to Azure AD. To resolve this issue, it is recommended for you to reset the password from Azure Active Directory admin center, then you can access office.com to update your own password without error!
For instructions, please refer to Let users reset their own passwords - Microsoft 365 admin | Microsoft Learn.
Should you need more help on this, you can feel free to post back.
If the answer is helpful, please click on “Accept answer” as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.
Thank you for your support and understanding.
Best Wishes,
Alex Zhang