The documentation at the head of commerce-order--admin.html template states that:
Available variables:
* - attributes: HTML attributes for the wrapper.
* - order: The rendered order fields.
* Use 'order' to print them all, or print a subset such as
* 'order.order_number'. Use the following code to exclude the
* printing of a given field:
* @code
* {{ order|without('order_number') }}
* @endcode
* - order_entity: The order entity.
However, order.order_number does not generate the order number through a customised template (generates a NULL).
After a bit of playing about found the following to work:
{{ order_entity.getOrderNumber }}
If this is a valid approach, can I suggest that the documentation within the template be altered.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Same problem as #5 in commerce-order.html.twig. I was able to get order_entity.getOrderNumber to work, though, so if anything, this should be documented in the twig file comments.
In commerce_order.module, perhaps the order number needs to be explicitly added to the variables array in template_preprocess_commerce_order?
{{ order.order_number }} should work if it's configured in the view mode. Have you tried that?