At least this has been my understanding which is why i favor a separate disk for linux and a separate disk for windows, and let the motherboard boot settings choose which disk to boot.
DUAL BOOT WINDOWS 10 AND LINUX ISO USB INSTALL
Windows on the other hand, is exclusive and only wants to know about itself and does not support any kind of dual boot, so if you install windows after having linux installed then windows is going to blow away the existing efi partition for itself and not preserve your linux install. What would happen is when the motherboard goes to run the boot loader on the efi partition, grub2 of linux is more than happy to say hi I see you have windows also let me put that in the boot menu for you as an N th option. If memory serves it was best to start with the disk having windows installed, and then installing linux afterwards and it was grub2 that was able to make it work. But with UEFI, and requiring one EFI partition on the disk, last I knew Microsoft Windows and Linux did not play that well. If it is an older laptop, with BIOS only and no UEFI, then I'll leave it to you to read up on MBR, but the concept is somewhat the same except no EFI partition. Assuming the laptop is modern and has UEFI it will want to see an EFI partition and a boot loader for it to execute to start a boot process and boot some operating system. So if you already have a system, having one disk, that disk will be partitioned in some manner. I'd like to make it dual-boot Manjaro and Windows.
You are asking, or have, two separate questions/problems happening.