Why does Windows Update say my device is missing security fixes and block the Windows 11 upgrade?

Why does Windows Update keep showing "Your device is missing important security and quality fixes" and refuse to offer the Windows 11 upgrade, even though PC Health Check says my laptop is eligible? I'm currently on Windows 10 Pro, version 1903, build 18362.1256, which I know is quite old.

I've already run the Windows Update Troubleshooter, stopped and restarted the Windows Update service, cleared the SoftwareDistribution folder, tried several Command Prompt fixes as administrator, and restarted multiple times. The same message keeps coming back.

Could this be because version 1903 is too far behind to upgrade directly, or is something in the update pipeline stuck? And what's the correct path to get this machine updated to a version that can actually move on to Windows 11?