I have recently been having this issue. I use Outlook locally. The issue seems to be that they changed how you click on things. If you single-click on the sender's name the email opens. If you click on the email but not on the sender it will highlight the email (like it use to). I REALLY REALLY REALLY hate this change. It is stupid!!!! It will cause the opening of emails that could be malicious that you want to highlight and put in the trash. Microsoft has made an extremely poor choice by implementing this.
Our IT department is reviewing this at the moment as it poses a security risk.
Microsoft you need to vet these types of changes better before implementing them.