One need to understand the relationship between the grub efi-loader and the grub.cfg
Despite the important connection between the two - they exist on two separate filesystems.
The grub config is found in /boot/grub/grub.cfg
And the efi-stub is located on a separate filesystem mounted at /boot/efi
EFI/Manjaro/grubx64.efiEFI/BOOT/BOOTX64.EFI
When you restore from a timeshift backup you may be restoring a grub.cfg which is not matching the efi loader.
This will create issues, where grub_shim_lock message is just one of them.
As already mentioned above the two loaders - despite the different names must be binary identical.
See the → [root tip] [recovery] Basic Manjaro Linux Recovery
When you have established a chroot - run the install-grub script - a Manjaro specific tool - designed to ensure correct function.
install-grub