I think /etc/default/grub miss an option

Support
1

Unless my new installation is not representative to a Nethserver installation (I install Nethserver over CentOS 7)

in /etc/default/grub

you have

GRUB_DEFAULT=saved

To make this effective you also need this option

GRUB_SAVEDEFAULT=true

which is missing

this have the behavior to boot and reboot on the last kernel you choose manually during the grub menu.
without the GRUB_SAVEDEFAULT=true it simply reboot on the default kernel and bypass the GRUB_DEFAULT=saved option

ref: arch linux - GRUB2: how to remember grub last choice - Unix & Linux Stack Exchange

2

RHEL/CentOS (and NethServer) is usually configured to boot the last installed kernel, not the last selected from the Grub menu.

IMO this is the correct behavior to always apply latests fix. Feel free to adjust your configuration as required by your use case! :wink:

See also https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/sec-customizing_the_grub_2_configuration_file

3

The settings of GRUB will be overwritten by a new kernel?