The Fields parameter is malformed and all field names supplied are converted to camelCase, causing the operations to fail because column by that name cannot be found. This issue has been raised on GitHub: Issue with New-MgSiteListItem · Issue #1185 · microsoftgraph/msgraph-sdk-powershell (github.com); and on answers @ docs.microsoft.com: Problem with New-MgSiteListItem's -Fields property (JSON body is being serialized into camelCase) - Microsoft Q&A
The problem came about in version 1.9.3 of Microsoft.Graph.Sites and persists through version 1.9.6.