Max,
MD04/ MD05 would not show the element if it cannot precisely determine the MRP element, this is standard behavior. In your screenshot the MRP element column is blank for the CustOrder line.
However, this issue has been discussed in the forum many times, you can use the following documents created by expert Caetano Almeida for a quick check in issues related to MD04.
MD04: Problems with sales orders
The issue mentioned here is discussed in point # 3 of the first document.
Best Regards,
Rajen