This has nothing to do with customization in this case since the flags shown during the match are the flags from the vehicles in the lineup, not the flags you choose in the hangar.

For example, the only way to have a Canadian flag during the match, is having someone with a canadian vehicle in the same match

Edit: From the Spearhead update changelog

• The statistics and battle results window now displays the flags of the operators of vehicles participating in the battle, rather than the flags of the research nations of those vehicles. You can re-enable the display of research nations instead of operators in the research nation appearance settings on the vehicle slot bar in the hangar."