Quite a few users are reporting that Outlook is not saving options such as Mark as Read options in the reading pane, Send Immediately, and Save sent items. When the user closes Outlook and reopens it, the settings are changed.
A new Option in Outlook to sync settings with the cloud is responsible.
To stop Outlook from changing the settings, go to File > Options > General. In the Cloud Storage Options, remove the check from "Store my Outlook settings in the cloud"
This option stores your settings in the cloud, so when you open Outlook on another computer, your settings sync.