I've looked in Preferences > Fields > Flags. I cant see Flag Format.
Flag Format is in the Define Layouts... window (the same window where you checked Flags for inclusion in the chart). Note that window is accessed by going to Charts / [the chart type you want] and selecting Define Layouts... from the Layout menu there.
I do see options there that are blank. I see the option, Narrative Form.
If you have Narrative form selected for Flag Format in the layout, you would need to have a Narrative Form for each flag. If you've selected one of the other two options for the Flag Format, it will just use the name of the flag.
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