Configuring preferences and options in Outlook.com
From our implementation, if accessibility services are turned on under Settings -> Accessibility, then only plain view for Conversations are supported. This makes it easier for users who need accessibility features to operate. If you’d like Conversation View to be enabled, please turn off your Accessibility services. Hope this helps.
What? Google's lack of concern for their broken email application brought me to Outlook. I get here and find out that an accessibility setting prevents threaded conversations in your app. This makes little to no sense. Half a year later and this can't be adjusted in one of your prime apps? I don't understand, but I guess I'll take it like a good little consumer.