The Bug
Banners created using legacy nbt have their color ids inverted, meaning that white is assigned to 15, orange is 14, black is 0, and so on. Here are some banners created using an online generator.
Illager Banner: http://www.needcoolshoes.com/banner?=pagvhfilhcawhqht
US Flag: https://www.needcoolshoes.com/banner?=pabBeD
Compare these with the banners I got in the screenshots. The illager banner screenshot has an actual illager banner from killing the petrol leader as a comparison.
Contact the creators of these tools and ask them to update them