I think I misunderstood something in my previous messages.
Anyway, your KDE Neon can see Windows because both are installed in legacy BIOS mode. Manjaro is installed in UEFI mode, so it can’t boot Windows. When you boot Manjaro through Neon’s grub, you are actually booting in BIOS mode (that’s why I asked if it worked).
I don’t know what solution you want to implement, but just you know that it is possible to convert a Windows BIOS installation into UEFI installation (for example: _ttps://www.tenforums.com/tutorials/81502-convert-windows-10-legacy-bios-uefi-without-data-loss.html)