If a date range field has a separator already defined before switching to the "AP Style" formatter, and it's not the value of "endash" nor "to", switching the Formatter to "AP Style" causes the separator select box to be red, and selecting a value doesn't make the error go away.
I suppose a potential solution could be to check if the value is something other than "endash" or "to" and default to one of the accepted values for the "AP Style" formatter.
Active
1.9
User interface