The need for a rebuild is related to the ‘Docker base image’ of Discourse, which is totally decoupled from the Discourse version number. We require it when there are critical changes to the OS-level dependencies inside the docker image.
So it would be tricky to include in the Discourse core release notes. But I do hear you on it being surprising/frustrating to not be able to update from the UI. Perhaps there are improvements we can make to the UI there.