So I did what you proposed.
I used manjaro-chroot -a from live ISO and executed those two commands.
Works fine at the first boot from HDD.
At the second boot I got these messages again:
error: no such device: here is a long hexadecimal number...
error: file '/boot/vmlinuz-5.10-x86_64' not found
press any key for continue
If I boot again from live ISO and use the manjaro-chroot the system is booting again from HDD.
No need the pacman commands to be executed, the boot is OK again.
This lasts until the next boot. Then I must to first boot live ISO and after that I can boot again from HDD.
What is the soluution?