For receiving the Feature Update(s) as update(s), you need to ensure that "Upgrade" category is selected in the classification tab of WSUS Server. Once done, you'll be able to see the feature update after next successful synchronisation.
Upgrade: Specifies an upgrade for Windows 10 features and
functionality. Your software update points and sites must run a
minimum of WSUS 4.0 with the hotfix 3095113 to get the Upgrade
classification. ------------// taken from "Configure classifications and products to synchronize".
Regarding your point that 1709 was available without this being checked, I highly doubt it. Maybe, those client systems were by default installed with Windows 10 version 1709. I'd request you to check with the concerned IT Team to verify this.
Also, just for your information, you can decline those updates (~upgrades from 7/8/8.1 to 10), and you can also set the option to download updates in WSUS only when you approve them. This would probably reduce the disk-space problem on the WSUS server.