Sharepoint is deprecating ACL. Please Validate that permissions were assigned properly read/write in the Sharepoint Site that has configured app permissions.
If not, permissions were assigned. Please ask to add permission read or write permissions to service principal using Permissions API or PnP are my recommended way due to sites.selected. Example:
- via Graph API /sites/{siteId}/permissions endpoint
- Grant-PnPAzureADAppSitePermission -AppId "yourAppID-75a7-47e3-bdb6-b763851c61b6" -DisplayName "TestApp" -Permissions FullControl -Site https://contoso.sharepoint.com/sites/projects https://pnp.github.io/powershell/cmdlets/Grant-PnPAzureADAppSitePermission.html