Oh, I used the bcdboot command
bcdboot d:\windows /s w: /f ALL
(‘w’ being the letter I assigned to an specific partition)
The first time I used this was when I accidentally deleted the boot partition a couple of years ago in an old pc, and did it in a separated partition.
This time, when doing it in it’s own partition, grub wasn’t able to find it regardless of which disk I was using to create the boot partition (It was still listed in bios). But somehow executing that command with ‘w’ being the boot partition for manjaro made it possible for grub to find the bootmgfw.