I think it's because grub-efi-amd64 is missing. Installing it in /target at the end of installation (after remounting the efi partition) fixes the problem.
** Changed in: grub2 (Ubuntu) Status: In Progress => Triaged ** No longer affects: livecd-rootfs (Ubuntu) -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to ubuntu-meta in Ubuntu. https://bugs.launchpad.net/bugs/1775743 Title: [regression] Cosmic daily images 20180606-11 install but boots only to grub prompt on EFI systems Status in grub2 package in Ubuntu: Triaged Status in ubuntu-meta package in Ubuntu: New Bug description: Cosmic daily images install successfully but then never boot (stuck in grub). I've tried a couple of machines. Fails: 20180611 Fails: 20180608 Fails: 20180607 Fails: 20180606 Works: 20180530 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1775743/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp