Additional System Center tools and components for managing infrastructure, applications, and services
Had a phone call with Microsoft. Has your software Center been updated to 2403 or higher? If so, this is a GPO setting! It can also be set locally to test out. Go to Computer Configuration --> Administrative templates --> Windows Components --> Windows Update --> Manage Updates offered from Windows Server Update Service (if this does not exist, you need new admx and adml files for the WindowsUpdate template). Enable the setting Specify source service for specific classes of Windows Update Service - then set the 4 options to Windows Server Update Services.
I had a few 25H2 that were built with ISOs, but not getting monthly updates even though SCCM was saying that they were compliant. Setting this let me see the monthly updates in Software Center and apply them successfully. On 23H2 systems, I now see the 25H2 upgrade as well, but this is now failing. Perhaps you can have more luck with it as I continue to troubleshoot.