With the Defiant boost, its Attack stage would be 0, or in other words normal. Since King's Shield drops its Attack by two stages and the Defiant boost increases it by two stages, they both basically cancel each other out.
So if you meant the Defiant boost when you said its Attack rose normally, you were incorrect. You must have read it incorrectly, or not realised that it wasn't the Defiant boost you read, because the Defiant boost would have said its Attack rose sharply.
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