Bootloader has forgotten how to boot from BSD-labels without MBR or GPT.
It is reproduced both clean install on the BSD-label, and when upgrading to 12.0
Replacing the loader from 12.0 with the loader from 11.2 (simple overwrite) solve problem.
After installing the system on BSD-label, before replacing the loader on the screen:
BTX loader 1.00 BTX version is 1.02
Consoles: internal video/keyboard
BIOS drive C: is disk0
BIOS 638kB/3603392kB available memory
FreeBSD/x86 bootstrap loader, Revision 1.1
Startup error in /boot/lua/loader.lua:
LUA ERROR: cannot open /boot/lua/loader.lua: no such file or directory.
can't load 'kernel'
Type '?' for a list of commands, 'help' for more detailed help.
1) booting from ISO FreeBSD 11.2, switching to the shell.
2) Mounting the root partition of the disk:
# mount /dev/ada0a /mnt
3)Copying the loader from 11.2:
# mv /mnt/boot/loader /mnt/boot/loader.12.0
# cp /boot/loader /mnt/boot/
After replacing the loader, the load is normal.