When formatting a FAT volume with ReactOS, you'll obtain a volume that cannot be mounted by MS FastFAT out of the box.
Indeed, by default, when we format as FAT, we zero jump instruction from the boot sector. MS FastFAT (and that's likely not the only one) expects jump instruction not to be null to ensure that's a FAT volume, and thus, fails to recognize the volume (and doesn't mount it).
This is hidden for ReactOS installation volume, because once boot loader is installed, jump instruction is properly set. This only applies to other volumes.
This also breaks installing ReactOS on a newly formatted volume with MS FastFAT: no jump instruction, it cannot mounted the volume, ReactOS cannot install.