the door was beautifully made by some local artisans....the diagonal wood piece is lighter than the rest of the door boards, and purely decorative on each side.... and doesn't affect the functionality of the door or how it will age. Also, the entire door is a lightweight pine.

No offence, but this is not a door built by professionals. There are fill marks all over the surface where nailed/screwed. The bracing, whilst it may not be structurally necessary for this particular door, is not just a decorative detail. Bracing on such a door is part of its historical functional design, it's to do with how load is transferred from the outer edge of the door to the inner to prevent the door dropping over time. It's all about making a quality door, honest in its design details.