HI Fabio - I came across this link having similar issues with "new" Outlook not marking messages as read.
It seems that Outlook now has a new option to change the "read message" settings which I am not aware existed before. Follow these steps:-
- Go to the gear icon in the top right corner, select mail, select "Message handling".
- Look at the options for "Mark as read" which can be tweaked.
- I found that the default setting on my Outlook was to "Mark displayed items as read after 5 seconds" which I found annoying so have changed.
- Make sure the setting matches your preference.
I hope this helps.