i am not interested in other z-programs as it does not help us and you in this case.
you can only compare with standard programs to be certain.
OSS note 411013 - Additional documentation on output determination in MM-IM
has a PDF with technical documenation attached, maybe it helps you further in analyzing the root cause.
At least it may give you an impression that we would need much more detailed information to analyze it from a distance.
OSS note 426554 - Output determination checklist for goods movements
has a checklist that you need anyway when you want open a call to SAP