Emailed being marked as Unread

I have an issue where all my emails from every sender is being marked as unread even after reading or replying. There are no custom rules set up on the account.
The setup is like this.
I’ve used my personal outlook email and synced it with my business outlook email so I could set the alias default as my custom work domain email. I also setup mail on my iPhone.
Has anyone else had this issue and how did you fix it?