why are the GEOM secondary GPT tables always corrupt?

[Chris H]

I've seen this discussed before, but there were so many
"solutions", I was left feeling this *must* be some sort
of bug in GEOM/gpart. So. I just blew away the tables on
a USB3 flash drive:

# gpart destroy -F da0

# gpart create -s gpt da0

# gpart add -t freebsd-ufs -l jails da0

# newfs -U /dev/gpt/jails

Added an entry to fstab(5)
OK this should be good to go. Mount, and umount
all return as expected, as does fsck(8).

Upon reboot, I receive the following:

/dev/gpt/jails: clean, 29961779 free (27 frags, 3745219 blocks, 0.0%
fragmentation)
GEOM: diskid/DISK-E600665E1DC77749: the secondary GPT table is corrupt or
invalid.
GEOM: diskid/DISK-E600665E1DC77749: using the primary only -- recovery
suggested.

But why?

This is on:
FreeBSD 12.0-CURRENT FreeBSD 12.0-CURRENT #0 r314700: amd64

Thanks for any information.

--Chris