This is - most likely - caused by the loader designated as the first in the systems firmware NOT being the Manjaro Linux efi-stub but the default grub which is not in sync with the available grub configuration.
It may be of interest to know why these things happen
The Manjaro Linux specific helper script install-grub is designed to handle this - the default GRUB and Manjaro Linux efi-stub must be binary identical to ensure they both understand the grub configuration.
In real world - this means
/boot/efi/EFI/Manjaro/grubx64.efi/boot/efi/EFI/boot/bootx64.efi
So to be binary identical - even the different names - the casing should be irrelevant but some firmware may expect a specific casing for the default entry.
sudo cp /boot/efi/EFI/Manjaro/grubx64.efi /boot/efi/EFI/BOOT/BOOTx64.efi
Technically - the FAT32 file system is case insensitive - but some firmware has a hardcoded path - where the casing matters.
Also on Linux the casing matters - but this is caused by rules applied on the mount point itself.
You can also find an excellent collection of troubleshooting tips on