Someone reported this previously - Wrong flag in zwift power - #2 by Paul_Allen
I wonder if your country is the same as them and if this is a bug with a particular country.
I think for issues with Zwiftpower you need to contact [zwiftpower@zwift.com] but just a warning that the product is “abandonware” pretty much, as I understand it the original developers left and Zwift seem to not have put anyone on looking after the product and are developing a replacement instead. So they may be unwilling or unable (or both) to fix your issue.
Good luck