By placing drawings on the banners it is possible to customize the shields with these same banners but unfortunately when making the combination the banner image is changed.

There are predefined figures for the banners (figure of the creeper, globe, among others) and even then the error remains.

The banner can be used to mark places on maps and used to be our flags but there is no way to use our symbol on shields with this error. 

Note: This error has been in Minecraft Java for years and so far has not been resolved (unfortunately) and persists so much that the feature has entered Minecraft Bedrock with this same error.