Cannot recreate an Ubuntu ISO that is bootable

I am trying to create a bootable ISO for a customised Ubuntu. The ISO I create is not bootable (it won't boot in VirtualBox). To debug I am not modifying Ubuntu, just trying to recreate a bootable ISO from the the original bootable Ubuntu ISO I downloaded from the web (it boots in VirtualBox). The steps are: 1. Mount original ISO 2. Make a read-write copy 3. Use genisoimage to create a new ISO The details are shown below. The new ISO won't boot. The file command indicates it is bootable, but does not contain a "DOS/MBR boot sector" (whereas the original ISO does). I have also shown some of the contents of the mounted disk (in case I am specifying the wrong boot catalog or image). Can anyone tell me what I am doing wrong?
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Steps 1 and 2. Mount original ISO and make a read/write copy:

> sudo mount -t iso9660 -o loop ubuntu-24.04.iso mntpoint
> mkdir mntpoint_rw
> rsync -a mntpoint/ mntpoint_rw

Step 3. Recreate the ISO:

> sudo genisoimage \,
   -o /tmp/custom.iso \
   -R -J -b EFI/boot/grubx64.efi -c boot.catalog -no-emul-boot -boot-load-size 4 -boot-info-table \
   $(pwd)/mntpoint_rw

I have tried various combinations of -b, -c options with files boot.catalog, EFI/boot/grubx64.efi and boot/grub/grub.cfg. Info about the original ISO (downloaded from web):

> file ubuntu-24.04.iso
ubuntu-24.04.iso: ISO 9660 CD-ROM filesystem data (DOS/MBR boot sector) 'Ubuntu 24.04.2 LTS amd64' (bootable)
>  ls -lh ubuntu-24.04.iso
-rw-r--r-- 1   6.0G Aug  1 13:37 ubuntu-24.04.iso

Info about the new ISO:

> file /tmp/custom.iso
/tmp/custom.iso: ISO 9660 CD-ROM filesystem data 'Custom Ubuntu-24.04' (bootable)
> ls -lh /tmp/custom.iso
-rw-r--r-- 1 root root 6.0G Aug  1 13:56 /tmp/custom.iso

Selected content of the mounted disk:

> ls -l --block-size=K mntpoint_rw
total 84K
dr-xr-xr-x 3    4K Jan 27  2025 EFI
dr-xr-xr-x 3    4K Jan 27  2025 boot
-r--r--r-- 1    2K Feb 15 19:15 boot.catalog
dr-xr-xr-x 2   12K Feb 15 19:15 casper
dr-xr-xr-x 3    4K Feb 15 19:09 dists
dr-xr-xr-x 2    4K Feb 15 19:09 install
-r--r--r-- 1   44K Feb 15 19:15 md5sum.txt
dr-xr-xr-x 4    4K Feb 15 19:09 pool
dr-xr-xr-x 2    4K Feb 15 19:09 preseed
lrwxrwxrwx 1    1K Feb 15 19:09 ubuntu -> .

> find mntpoint_rw/EFI -type f | xargs ls -lh
-r--r--r-- 1   945K Jan 27  2025 mntpoint_rw/EFI/boot/bootx64.efi
-r--r--r-- 1   2.3M Aug  1 13:56 mntpoint_rw/EFI/boot/grubx64.efi
-r--r--r-- 1   837K Jan 27  2025 mntpoint_rw/EFI/boot/mmx64.efi

> find mntpoint_rw/boot ! \( -name '*.mod' -o -name '*.lst' \) -type f | xargs ls -lh
-r--r--r-- 1   2.4M Jan 27  2025 mntpoint_rw/boot/grub/fonts/unicode.pf2
-r--r--r-- 1    583 Feb 15 19:13 mntpoint_rw/boot/grub/grub.cfg
-r--r--r-- 1   7.5K Jan 27  2025 mntpoint_rw/boot/grub/i386-pc/efiemu32.o
-r--r--r-- 1    11K Jan 27  2025 mntpoint_rw/boot/grub/i386-pc/efiemu64.o
-r--r--r-- 1    31K Jan 27  2025 mntpoint_rw/boot/grub/i386-pc/eltorito.img
-r--r--r-- 1    318 Feb 15 19:13 mntpoint_rw/boot/grub/loopback.cfg
-r--r--r-- 1   145K Apr  9  2024 mntpoint_rw/boot/memtest86+x64.bin