I get these log lines as well every time I open the Blueprints menu dropdown.
When I debugged it I found that it happens when the menu items concerning the GameMode are created.
It uses FText::Format() with e.g. the format string “GameMode: Edit {GameModeName}” but seems to expect a numeric parameter like “{0}” instead of the “{GameModeName}” named parameter. However, after printing the formatting warning, it then uses a numeric parameter as fallback and thus it still works.
I took a look at the 4.13 source and here it is replaced with e.g. “GameMode: Edit {0}”. So it seems to be fixed in 4.13. 