I remember a case years ago when we used a custom program to trigger collective outputs, that when it detected a problem with a completely different output type in a preceding delivery in the same shipment, the outputs (I was using special function 8 as transmission medium for my output type) in the next delivery was discarded from processing. We had to modify our program to issue warning instead of error in such situation and then all deliveries were processed successfully.
If the process works in ECC, when you post manually from VT02n, then the note will not help you much, unfortunately.
If it is not a bug in the z-module, then your best bet is to check for performance problems and errors/warnings in the shipment log.