boot – How to prevent a UEFI entry for bootable USB flash drive disappearing from BIOS when UEFI entries updated by Linux on fixed drive?

I have my main Arch Linux install on my SSD (Main-Linux) and a backup installed on a removable USB flash drive (Backup-Linux) which is bootable.

However when I update the UEFI boot loader in Main-Linux without the USB flash drive attached Backup-Linux's entry in BIOS is removed and the BIOS UEFI boot manager won’t show the Backup-Linux option despite having USB flash drive boot enabled.

The USB flash drive has it’s own EFI partition and bootloader installed.

If I update the bootloader from Main-Linux while the USBUSB flash drive is plugged-in it detects it and adds it to the options, but if my Main-Linux breaks for some reason I need my system to boot from the USB but I’m afraid it won’t.

So two questions, I guess:

How can I instruct GRUB to keep the USB flash drive boot option when updating the boot loader even if the USB is not attached.
Why is my BIOS not detecting the UEFI boot loader on the USB flash drive? Is the bootloader on the SSD somehow blocking it once it has been detected?