Decided to switch to systemd-boot today* after this adventure and ran into this error. Not much on github (other than this issue) or the forum other than this thread.
Sounds like adding filesystems."/boot".options = [ "umask=0077" "defaults" ]; is the recommended fix?
EDIT: * for context this is on a zfs-root UEFI system previously using grub
EDIT2: Added "defaults", in keeping with the default value if options is omitted entirely