That is what I was starting to think, but I managed to get the Debian 8.9 SD card from the BBW to boot in the BBG by editing /boot/uEnv.txt:
##BeagleBone Black/Green dtb's for v4.1.x (BeagleBone White just works..)
##BeagleBone Green: eMMC disabled
This card has been in the BBW since 2015.
This issue is moot now since I've got the original Debian 8.9 SD card from the BBW to boot on the BBG and its working fine again, but someday I may need a more "modern" system if I add new features so I'd like find a solution.
This is an "early" model BBG that required an eMMC flash to do anything useful, obviously the old image in the eMMC won't boot with my hardware attached as I'm using gpio pins on J8 that require the eMMC be disabled.
So I think there must still be an issue in the boot up with the Debian 9.2 image and at least some of the older BBG boards.
Unfortunately there is no practical way to plug in a usb-serial adapter to monitor the bootup with my hardware attached -- its an IOT application and unfortunately physical access is very difficult.
Just for grins, I put the bone-debian 9.2 SD card /boot/uEnv.txt back to stock settings and tried it in my old Rev A5A BeagleboneBlack that has been collecting dust. It booted fine and the GUI came up, I thought I'd downloaded an IOT image without GUI. The GUI is too sluggish to be useful, perhaps is usable with ssh -X, but pcmanfm didn't show icons when I started it from an ssh -X session, although it otherwise seemed to work.