By nisitiiapi at ameriDroid Forum
Instead of burning an image directly to eMMC, I burned to microSD and installed to eMMC from microSD (using write2emmc in debian). After that, I have been able to consistently boot from microSD first even with different images (tried original debian on microsSD that I had burned to eMMC and also armbian cinnamon, both successful boot from microSD with OS on eMMC).
So, it may be necessary to install to eMMC from microSD to enforce the boot order. Not sure if that is unique to the debian install I was using or not, but may help others.