The Empty Birdcage | Drowsy Puzzles

An empty birdcage hangs by the window, where calm afternoon light slips inside.

The old man watches its shadow on the wall.
For some reason, the shape is different from the cage itself. In the center, he almost sees the faint shadow of a small bird. It feels as if a familiar chirring, a soft beat of wings, might reach him.

But there is no time to linger.

He turns his attention to what feels irregular within the shadow.
To wake from this gentle illusion, it seems he must find it.

　

Puzzle:
Which part does not follow the rule?

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Hint:
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There is a rule linking the number of patterns at the top and bottom with the design in the middle.

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Answer:
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When the number of patterns at the top and bottom is even, the middle section is plain.
When it is odd, the middle section contains a white cut-out shape.
The place that does not follow this rule is the answer.

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This work belongs to the collection
Drowsy Puzzles — A Quiet Afternoon by the Window. (Complete digital set of 4)