Using classic Outlook for Windows in business environments
When you turn the reader off, do you do it when you are composing mail then send it? Try turning it off then cancel the send. (This works for for other settings - like zoom levels.)
Did you try turning it off then immediately restarting outlook?
Do you have "Store my Outlook settings in the cloud (requires restarting Outlook)" as at option at the top of File > Options > General ? If yes, it should be unchecked- then restart outlook. Try it for a few days then try checking it again.
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I also noticed two entries for Microsoft 365 in my Installed Apps: Microsoft 365-en, no size listed; and Microsoft 365 (Office), 99.1MB. Why do I have two; and would this influence this behavior in Outlook?
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Nether app would affect it (nor new Outlook). The 99MB file is the Office "dashboard" app. The one with the language code (en in your case) is the office desktop software.
The Microsoft 365 in this screenshot is the dashboard app - the office apps in the screenshot are from the Microsoft 365-en file.