lp:maas-images downloads all the bootloaders MAAS needs and puts them in the MAAS stream at images.maas.io. It does this on an AMD64 host for all architectures. For ARM64 and PPC64 it must generate the bootloader using grub-mkimage.
Since the release of 2.02-2ubuntu8.3 grub-mkimage fails if the prefix isn't specified with -p. Previously I did not have to specify -p to use the default.
$ grub-mkimage -o bootloaders/bootloaders/uefi/arm64/20180830.0/grubaa64.efi -O arm64-efi -d /tmp/maas-images-zj82hpij/usr/lib/grub/arm64-efi/ -c /tmp/maas-images-zj82hpij/grub.cfg gcry_rmd160 crypto affs gcry_idea lsefisystab test extcmd testspeed fat minicmd terminal lzopio exfctest normal cpio serial gfxmenu gcry_camellia ufs1_be part_plan datetime videotest iso9660 functional_test cmp_test fdt part_gpt lsefi part_sunpc raid5rec adler32 part_apple bitmap ntfscomp search_fs_uuid acpi gcry_dsa video_fb search ctz_test keystatus mmap mdraid09_be minix3_be syslinuxcfg mdraid1x setjmp videotest_checksum gcry_md5 progress part_dfly msdospart nilfs2 gcry_twofish gcry_des lvm gcry_serpent loopback test_blockarg echo signature_test pbkdf2 hfspluscomp gcry_tiger parttool lsmmap diskfilter ldm mul_test xnu_uuid_test gcry_whirlpool bfs file gcry_sha1 chain ext2 macho part_bsd ntfs gcry_rsa offsetio jpeg priority_queue romfs lssal xzio png tr linux disk gfxterm_menu memdisk afs efinet zfs ufs2 minix blocklist geli setjmp_test gfxterm_background eval xfs cryptodisk read minix2 archelp font efi_gop sleep halt hashsum part_sun password hfs raid6rec net boot datehook dm_nv ufs1 part_msdos reiserfs search_label sleep_test zfscrypt date gfxterm gcry_arcfour ls lsefimmap search_fs_file gettext part_acorn gcry_sha512 videoinfo help efifwsetup gcry_sha256 lsacpi part_amiga udf video_colors cpio_be password_pbkdf2 luks mpi true elf video verify minix2_be crc64 jfs procfs btrfs minix3 bitmap_scale tar regexp gcry_blowfish bufio all_video gzio time cmdline_cat_test zfsinfo mdraid09 gcry_crc gcry_rfc2268 cbfs odc hello xen_boot part_dvh tga div trig gcry_cast5 gcry_rijndael minix_be squash4 terminfo configfile shift_test fshelp loadenv scsi gcry_seed testload pbkdf2_test reboot hexdump gptsync xnu_uuid hfsplus newc gcry_md4 cmp memrw sfs exfat div_test http macbless cat probe bswap_test tftp
Prefix not specified (use the -p option).
Usage: grub-mkimage [OPTION...] [OPTION]... [MODULES]
Try 'grub-mkimage --help' or 'grub-mkimage --usage' for more information.
$
Since you say this problem began with 2.02-2ubuntu8.3, setting the regression-update tag.