Thank you for documenting this so carefully and for explaining why it happens. I have done a few of these AS+DE+LH flights this year and this both solved my problem and feel like I have a little more insight into this crazy system.

It's not really a small thing! This is important so that I can add my MP number, add my TSA KTN, and pick a seat assignment on the AS flight. CSRs at DE will tell you need to do this all at the airport it's a pain to have to wait in line at the airport to do these and the seat assignments are often taken if you. Thanks again!