Wow. I guess the Alexa list integration is broken as of today, July 1, 2024.
Warning: Starting July 1, 2024, you will no longer be able to use List skills or the List Management REST API to access Alexa lists, i.e., the Alexa Shopping and To-Do lists, in your skills or apps. For other ways to build custom voice experiences, see Steps to Build a Custom Skill. Please contact us if you have any questions.