Custom Ubuntu based iso won't boot if "USB Legacy Emulation" is disabled

I have a custom Ubuntu based iso which boots fine in BIOS mode on a Toshiba C55-A5310. It will also boot in UEFI mode if "USB Legacy Emulation" is enabled. With the machine is in UEFI mode and "USB Legacy Emulation" is disabled, the USB is not recognized. Here's the code I used to build the fat partition within the iso:

#! /bin/sh

############################
#   Make UEFI partition    #
############################

BOOT_IMG_DATA="$PWD"
BOOT_IMG=efi.img

mkdir -p $(dirname $BOOT_IMG)

truncate -s 4M $BOOT_IMG
mkfs.vfat $BOOT_IMG
mkdir -p $BOOT_IMG_DATA/EFI/boot

grub-mkimage \
    -C xz \
    -O x86_64-efi \
    -p /boot/grub \
    -o $BOOT_IMG_DATA/EFI/boot/bootx64.efi \
    boot linux search normal configfile \
    part_gpt btrfs fat iso9660 loopback \
    test keystatus gfxmenu regexp probe \
    efi_gop efi_uga all_video gfxterm font \
    echo read ls cat png jpeg halt reboot

mcopy -i $BOOT_IMG -s $BOOT_IMG_DATA/EFI ::

Here's the code I used to build the iso:

############################
# Create the new ISO image #
############################

# The example names get mapped to their roles here
orig_iso="$HOME"/foxclone/foxclone025-01.iso
new_iso="$HOME"/foxclone/foxclone025-02.iso
new_files="$PWD"
mbr_template=/usr/lib/ISOLINUX/isohdpfx.bin

# Extract MBR template file to disk
dd if="$orig_iso" bs=1 count=432 of="$mbr_template"

xorriso -as mkisofs "$new_files" \
  -o "$HOME"/foxclone/foxclone025-02.iso \
  -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin \
  -c isolinux/boot.cat \
  -b isolinux/isolinux.bin \
   -no-emul-boot -boot-load-size 4 -boot-info-table \
  -eltorito-alt-boot \
  -e boot/grub/efi.img \
   -no-emul-boot \
   -isohybrid-gpt-basdat \
  -m grub.cfg
  "$new_files"

Here's the result of fdisk -lu

larry@larry-Satellite-C55-A:~/foxclone$ fdisk -lu foxclone025-02.iso
Disk foxclone025-02.iso: 549 MiB, 575668224 bytes, 1124352 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x1e64812c

Device              Boot Start     End Sectors  Size Id Type
foxclone025-02.iso1 *        0 1124351 1124352  549M  0 Empty
foxclone025-02.iso2        288    5215    4928  2.4M ef EFI (FAT-12/16/32)

Any help is appreciated