For some reason, the KF8 version indeed doesn't get a cdeKey attributed, even an old-style hash. That's... weird?
(And that field being NULL is screwing with quite a number of things, BTW).
Potentially a FW 5.6.x change? (I checked on a 5.7.2 PW2). I can't really do anything for that. After all, the name of the field is pretty accurate, it's key

.
I'm fairly sure that untouched, KindleGen produced dual mobis DO get one. So, err, very strange.
I don't recall if I ever did some testing with KF8 files split from dual-mobis, but I *think* I did, and that worked fine.
So, meh? If you can reproduce the issue with another book using the same workflow, it's either a bug somewhere in the chain of said workflow, or the Kindle framework not dealing with it well (in which case, I'd go back to: bug in the workflow, just assign a random uuid as the ASIN, like Calibre does, because that works).