Hi,
I have configured WSUS, in Products I have checked "Windows 10" and my Windows Servers 2012, 2016, 2019.
Now I see that on WSUS page neither critical nor security updates are available,
when I click on client computer check updates then I have System is Up to date.
But when I click check updates in Windows Update Online, then computer start to download new security patches.

Second example:
Windows 2012 R2 not connected to WSUS, report now that two security patches are ready to download:
Windows Server 2012 R2 connected to WSUS do not have any updates available: