It was a deliberate change in the 2.1 scheduler, as there was a lot of demand for mixing reviews, and a single general subject with lots of subtopics is common. For now, you’ll need to downgrade to the old scheduler using an older Anki version if this is a dealbreaker for you.
There are no technical reasons why the v3 scheduler couldn’t support this case, so I will look into adding that as an option in the next alpha.