I have the latest Fast Insider build of Outlook and I am also having this problem with text being invisible. It is also happening in Excel spreadsheet cells for me.
Skywatcher17
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Yes, the text is selectable and copy/pastable...I have been pasting into TextEdit in order to read emails. Since I originally posted, I've also noticed it happens in the Mail app, but only when composing a new message - reading received messages is fine.