OK I think -- think -- it might be working properly now. I did a few things and I'm not sure which of them was responsible.
a) I ran Gparted and set the "boot" & "esp" flags on the partition.
b) I ran Gnome Disks and checked the "Legacy BIOS Bootable" flag.
c) At boot time, I selected to boot from sda from the BIOS.
I was able to successfully boot into my LFS system twice, and then boot back into my LM 21.1 host system.
I'm not totally ready to mark this solved but it's definitely progress. I'm going to keep playing around with it. I guess one of those flags was the problem? Seems odd but just happy that it sorta kinda works. If I have to keep selecting this partition from the BIOS at boot time, that's OK with me but wish I knew why my GRUB settings didn't work.

at the grub menu press c for grub> prompt, what is output of
If you see the lfs kernel, try this to boot at the grub prompt

1 members found this post helpful.

I didn't see your last post before posting my lost post, maybe there is a setting in the bios that needs to be changed.

Is sdb first in the boot order in the bios/firmware settings?

Marking this thread as solved since I can repeatedly get into my new LFS system, even though it takes an extra step.
- at bios boot time, select to boot from sda instead of sdb
- grub menu comes up even though grub is installed on sdb mbr
- select the lfs menu entry
Thanks for your help, colorpuple.

you're welcome