And if you “view” the tag in album browser you will see all the Bowie albums in your library besides the non-Bowie albums from the Tag. You will most likely even get results if you load the Tag in the composition browser, even if you have never added a specific composition to the Tag itself. That is how Tags are designed to work in Roon. While you can Tag many different things (objects) in Roon, mixing object types in a single tag may make result sets somewhat unpredictable for the user.
So for your use case: Either create a “Discover Artists” that only contains artists or use your existing tag to jump to the stored content within (don’t open that Tag in any other browser).
Roon shows you exactly the items you tagged if you look into the tag. Apply the tag on any other browser though, Roon will try its best to make the tag work in that browser, no matter what objects you included. Example: You create a tag which contains compositions, Roon will show you all the albums in your library that contain any (known to Roon) performance of the compositions from the tag if you load the tag in the album browser. If you load it in the artist browser, you may see the composers as well as all the main performers for the selected composition’s performances in your library.
What you really want to do is combine tags and bookmarks.
If you focus on the tags “Discovery Artists” and “Discovery Albums” and make that a bookmark called “Discovery” then the bookmark will get closer to what you are looking for. You could add other items to the bookmark as well. Albums in your library you haven’t listened to, for example.