Totally agree. According to a team member in this post: Bases grouping (by property of list type) - Help - Obsidian Forum, this is not a bug, but the intended behavior.
There may be use cases for the current behavior, but I cannot think of one. On the other hand, there are many instances where I need the proposed behavior. If I am looking for movies of the Comedy genre, I want to look in the Comedy group regardless of whether the movie was also a Romance.
The only workaround is to use dataview which can split a list of values into separate groups. But then you lose the speed and editability of bases.