I too have this issue since updating to 11.4 and hope it can be fixed soon.
Prior to 11.4, all of the Apple TV 4k audio would be routed to the HomePod when selected as the audio device.
Since updating to 11.4 almost all of the audio is instead routed over HDMI to the TV.
Netflix movies and Netflix autoplay trailers in the app interface actually play on HomePod with 11.4
Everything else, including the ATV interface (i.e. swipe sounds), Hulu videos, etc., play through HDMI instead.
It is very frustrating when a Netflix video ends and it immediately transfers audio to the TV when exiting full screen for the 'play next' options.
With version 11.3.x everything worked fine
Please Apple fix this!