It might also be because you are sorting by Subject. In New Outlook there is no way to disable "Group by Subject" when you Sort by Subject.
You can Vote for and Comment on this Feature Request if you want the ability to Sort without auto Grouping.
https://feedbackportal.microsoft.com/feedback/idea/27de2f9f-3cfa-f011-bb46-7c1e529117fa
(You must "Sign In" at the top right of the Feedback page before you can vote).