Hi Jeff Mills,
As per your description, it seems issue only occur with specific SharePoint online custom list using the Modern list view. However, if you switch to classic view all my items are displayed in the view. May I know if my understand is correct? If so, this is really abnormal behavior.
Based on my test result after creating a custom list it seems I cannot meet same behavior as lie yours. See image below for your reference:

Have you tried to edit the current view for columns and see if it displays any list items? How about the libraries under the same site? Technically, the library is a special list in SharePoint.
After search and based on my experience, this issue related with some different columns type you created in your custom list and some users fix it by remove the JSON code of column and the items are back. You may also try and check if this also help for you.
Meanwhile, could you please share with us your column type in your custom list. Also if you don't mind could you please confirm if you are using any custom column in your SharePoint list.
I appreciate your understanding and stay safe!!
Best Regards
Waqas Muhammad