Did you maybe overwrite the content of the /boot/efi partition in which the Windows bootloader was? (In the installer, there is a check box where you can select ‘Format’ (or not to)).
I’m in a similar position. Reinstalling Fedora 33 with an existing Windows 10 installation.
In my case, Fedora is on a different physical drive than Windows. Both Windows and Fedora are EFI
Start: Windows HD unplugged.
Install Fedora
Update and check Fedora
Shutdown machine and plug in Windows HD
Restart machine, check BIOS to see that UEFI “sees” Fedora and Windows. Yes.
Restart machine, boot to Fedora. Windows doesn’t show up in the Fedora boot options.
Login to Fedora. Become root (or use sudo).
Navigate to location of grub.cfg (/boot/efi/EFI/fedora)
cp grub.cfg to grub.cfg.old
run ‘grub2-mkconfig > grub.cfg’ in /boot/efi/EFI/fedora
Watch output from this command. something like:
Found Windows Boot Manager on /dev/sda5@/efi/Microsoft/Boot/bootmgfw.efi
(Note: In this case Fedora is on /dev/sdb{x}, Windows on /dev/sda{x}, and sda5 is where the Windows EFI partition is).
Logout
Reboot machine
Windows menu entry should appear.