Do man shutdown :
OPTIONS
-r Requests that the system be rebooted after it has been brought
down.
-h Requests that the system be either halted or powered off after
it has been brought down, with the choice as to which left up to
the system.
-H Requests that the system be halted after it has been brought
down.
-P Requests that the system be powered off after it has been
brought down.
Como você pode ver, as opções -H e -P desligarão seu sistema. E o sudo reboot só reinicializará o seu computador, porque foi projetado dessa maneira.
Para reiniciar o seu sistema, por favor, desative-o e posso recomendar sudo shutdown -r e sudo init 6 . ⬅ ⬅Isso não deu certo para mim @biohazard】