Totally broken system after December 2025 Stable Update

Yes.
Adapt the command.

On my system, the directory is named “boot” - not: “BOOT”
and the other is not named “manjaro” but “Manjaro”

lower case vs upper case matters.
if the directory doesn’t exist under the name BOOT or manjaro, the command will fail and you’d see that.

On my system, the command would be:

sudo cp /boot/efi/EFI/Manjaro/grubx64.efi /boot/efi/EFI/boot/bootx64.efi

sudo tree /boot/efi/
[sudo] Passwort für jo: 
/boot/efi/
└── EFI
    ├── boot
    │   └── bootx64.efi
    └── Manjaro
        └── grubx64.efi