I’m not sure if it’s because the 2018.4 LTS channel is being set up, or if the Unity Hub is just not seeing builds in 2018.4.x. For other releases, Unity Hub 2.0 beta has shown the build almost immediately after they appeared on the official download page.
You must wait a few hours before it shows up. 
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On the 1.x hub it would often take several hours to show up, but on 2.0 beta new builds were popping up almost immediately - at least for me. When this one didn’t appear for a couple hours I figured it was worth posting about. I’m back in from a meeting, and it’s there now. Cheers