I've processed a few returns on different blushes and pressed powder products for the same reason. it's likely that the brush you're using, depending on how you wash it, is holding onto water/shampoo/oils/cream face products and when you hold the brush upside down or at an almost upside down angle, it shifts to the tip of your brush, and allows the residue to settle into your blush/powder etc. if you're shampooing your brush with soap and water; mix a little bit of alcohol into it to help it dry faster, and make sure your shampoo is clarifying like a baby shampoo or detoxifying cleanser, that way it doesn't leave any buildup in your brush.

HTH!

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