Well, it seems somehow "share_not_sync: true" -- I must've set it a LONG while ago, but I've no idea why.
According to Java Mobi Metadata Editor:
The old book(s) has
no exth fields 113, 501, or 504.
The new one(s) has 113:$calibre_uuid, 501:EBOK
I'm not sure why old has no cdetype, (before I touched that setting) nor why the new has an ASIN if that was disabled???
But I can fix the problem books by resending with the new settings or by adding in the cdetype=PDOC/EBOK (either works) using JMME.
So clearly something wrong on my setup.
Thanks for the explanations.

Fascinating stuff.
Now excuse me, I need to go tinker with something.