We are using Configuration Manager current branch to update the existing Windows 10 machines with quality updates. All Configuration Manager Components are running on ‘Server 2016’ + KB4284833. The error is reproducible in CMCB 1802, 1802 + KB4339794 or TP 1806.2.
Windows 10 cumulative updates categorized as ‘Updates’ are not imported in to Configuration Manager WSUS, e.g. KB4284848:

However, Windows 10 cumulative updates categorized as ‘Security Updates’ are imported, e.g. KB4284835:

‘Critical Updates’ like KB4338853 are imported too:

SUP properties are set to import ‘Updates’:

If we try to import the updates manually in WSUS, it fails due to incompatible WSUS version:

This brings us close to the root cause: WSUS Catalog import failures (March 2, 2018) “We are currently working on a known issue with importing updates on WSUS 5.0 (Windows Server 2016) from the Microsoft Update Catalog, which fails with the following error: “This update cannot be imported into Windows Server Update Service because it is not compatible with your version of WSUS””

Using the workaround of switching to Protocol=1.8, we could import the updates in WSUS, so they showed up in CMCB to be handled by ADRs and installed on the clients.

Since this issue is still not fixed, use this as a reminder to check the updates in your Software Update Groups, especially if you use ADRs.