Hi Scott,
The default dimensions from the product are only used on e.g. the sales order line if the specific value is not filled.
The flow is:
Sales order header is taking the dimensions from the customer.
Then it is possible to update values if you need to do so.
The sales line is then taking the dimensions from the sales order header.
When you provide an item, the dimensions which are not filled are taken from the released product.
If you need to have the dimensions taken from the item, you have to ensure it was not filled at the customer or the sales header.