The Illager Banner has an undetailed texture with flat colors instead of looking like it's made of actual banner patterns.
[media]See pictures for comparison. The default banners have a more detailed texture with a different color palette.
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Beta 1.12.0.6 changelog mentioned "Updated illager captain banner texture". I'm not sure if this is the issue that was in the changelog.
Edit: fixed in beta 1.12.0.10 the banner now has the textures of how it should look like in the Java Edition
I think that's because the Illager/Ominous Banner is a separate type of banner in the Bedrock Edition, unlike the Magenta Banner with the Black Inverted Chevron and Black Chevron patterns.