EFI Not Installing Correctly
Regarding Lenovo_ThinkPad_X1_Carbon_(Gen_2)#UEFI, this was demonstrated on 2 different ThinkPad X1 Carbon computers. When the EFI boot option was created with
efibootmgr (Unified Extensible Firmware Interface#efibootmgr), it would disappear when the flash drive (archiso) was removed.
I'm not entirely sure why this was happening since the EFI boot options should be stored on the MOBO nvram, not the flash drive. I was able to work around it by installing the option from the EFI shell, as described in here.