Let me make an educated guess.
Our company offers a consulting service for testing DICOM interfaces. Mainly modality manufacturers are doing this with us. Not transferring the Accession Number from worklist results to acquired images is something that we find in almost every project. It seems that there is a very common misconception of what a DICOM type 2 attribute is.
What it is: "You may leave it empty if you do not know a correct value for it" -> explicitly conveying the information "I do not have that information"
How many implementors read it: "You may leave it empty if you want"
Obviously, when the modality receives the Acc# via worklist, it cannot claim not to have the information, but this seems to be easy to miss.
I would bet one box of finest franconian beer (shipping not included) that it is a bug in the modality. :-) If you would like assistance in verifying and obtaining evidence for this, you are welcome to PM me.