If you see inside of an Ominous Banner if you are near water or anything like black lines of other Ominous Banners, Nether Portal blocks, End Portal blocks, players, mobs or particles like the campfire smoke you will see it through the banner. I don’t know if with any other banner happens the same but I assume it will happen the same.
Here an image of me inside of an Ominous Banner with some of this things mentioned
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We are tracking this issue in MC-167532, so this ticket is being resolved and linked as a duplicate.
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