If you are Windows 10, version 1809, or Windows 2019 1809 user/admin and WSUS still haven’t detected update KB5003171, then you are not alone. Update KB5003171 won’t be detected until you install Servicing stack update KB5003243.
Microsoft Update release note mentions the following:
You must install the May 11, 2021 servicing stack update (SSU) (KB5003243) or later before installing the latest cumulative update (LCU). SSUs improve the reliability of the update process to mitigate potential issues while installing the LCU and applying Microsoft security fixes. For general information about SSUs, see Servicing stack updates and Servicing Stack Updates (SSU): Frequently Asked Questions.
If you are using Windows Update, the SSU will be offered to you automatically. To get the stand-alone package for the latest SSU, search for it in the Microsoft Update Catalog.