When looking through the gaps in wooden and iron doors, the door looks flat. However when looking through the gaps in the acacia and jungle doors, the door looks 3D. The images below show the issue.
To reproduce a similar situation for yourself:
Put a wooden door next to an acacia door and look through the gaps in the doors.
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That's not a bug. They specifically didn't change the original doors on purpose. They didn't want to mess with the classics to avoid angering people.