UEFI boot hangs after loader

Just upgraded my Asus UX303L (amd64) from 11-STABLE to 12.0-BETA1 r339529
by source.  Have a problem with booting which hangs after:

>> FreeBSD EFI boot block
 Loader path: /boot/loader.efi

Initializing modules: ZFS UFS
Probing 5 block devices ... done
  ZFS found the following pools: zroot
  UFS found no partitions
  Consoles: EFI console
  FreeBSD/amd64 EFI loader, Revision 1.1

    Command line arguments: loader.efi
    EFI version 2.31
    EFI Firmware: American Megatrends (rev 4.654)
    Console: efi(0)
    Load Path: HD(4, GPT [ ... ]
    Load Device: Pci Root [ ... ]
    Boot Current: 0001
    Boot Order: 0001 [x]
    Boot Info Path: HS(1, GPT,  [ ... ]  /\EFI\BOOT\BOOTX64.EFI
 -

So it gets into loader.efi which runs but stops I think just before loading
the kernel.  Partitions:

=>       40  250069600  ada0  GPT  (119G)
         40       1600     1  efi  (800K)
       1640       1024     2  freebsd-boot  (512K)
       2664       1432        - free -  (716K)
       4096    4194304     3  freebsd-swap  (2.0G)
    4198400  245870592     4  freebsd-zfs  (117G)
  250068992        648        - free -  (324K)

and the EFI partition is FAT 12.

I can't provide (at the moment) any output from efibootmgr:

root_at_gryphon:~ # efibootmgr -v
efibootmgr: efi variables not supported on this system. root? kldload efirt?
root_at_gryphon:~ # kldload efirt
kldload: can't load efirt: module already loaded or in kernel

which I don't understand.

I'm going to rebuild the kernel (currently GENERIC) and see if that allows
me to load efirt / use efibootmgr.

In the meantime, I should be very grateful for any advice.

( Currently booting using a 11-RELEASE memstick image and dropping into its
loader to get to the installed 12-STABLE ).

/Harry