Apple has released tvOS 15.5.1 and HomePod Software 15.5.1 to the public with bug fixes and to address an issue where music could stop playing after a short time.
The operating system for Apple TV and HomePod are rarely updated on their own, but Apple has released a bug fix update to address specific issues for those platforms. The previous updates for these platforms were released on May 16.
The release notes for the HomePod Software update say it addresses an issue where music could stop playing after a short time. Since the HomePod software and tvOS are essentially the same release and share a build number, Apple released updates for both operating systems.
The Apple TV, HomePod, and HomePod mini will update automatically for most users. However, if you'd like to force the update right away, just navigate to the Settings app on Apple TV for tvOS or to the Home app for HomePod Software.