sudodus@bionic64 /media/multimed-2/test/ubuntu $ sudo gdisk groovy-desktop-amd64_2020-10-08.iso
[sudo] lösenord för sudodus: 
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present


***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format
in memory. THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by
typing 'q' if you don't want to convert your MBR partitions
to GPT format!
***************************************************************

Exact type match not found for type code CD00; assigning type code for
'Linux filesystem'

Command (? for help): q
sudodus@bionic64 /media/multimed-2/test/ubuntu $ sudo gdisk groovy-desktop-amd64_2020-10-08_fixed-by-xorriso.iso
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): q
sudodus@bionic64 /media/multimed-2/test/ubuntu $ sudo gdisk groovy-desktop-amd64.iso
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): q

sudodus@bionic64 /media/multimed-2/test/ubuntu $ ls -l groovy-desktop-amd64_2020-10-08.iso groovy-desktop-amd64_2020-10-08_fixed-by-xorriso.iso groovy-desktop-amd64.iso
-rw-rw-r-- 1 sudodus sudodus 2931490816 okt  9 11:36 groovy-desktop-amd64_2020-10-08_fixed-by-xorriso.iso
-rw------- 1 sudodus sudodus 2931951616 okt  8 13:02 groovy-desktop-amd64_2020-10-08.iso
-rw------- 1 sudodus sudodus 2936969216 okt  9 21:15 groovy-desktop-amd64.iso

sudodus@bionic64 /media/multimed-2/test/ubuntu $ md5sum groovy-desktop-amd64_2020-10-08.iso groovy-desktop-amd64_2020-10-08_fixed-by-xorriso.iso groovy-desktop-amd64.iso
0ca0a6242caffcb7c67497fc659f2b8c  groovy-desktop-amd64_2020-10-08.iso
d585d0bffe1161302c000b058482a53a  groovy-desktop-amd64_2020-10-08_fixed-by-xorriso.iso
00cbcce8855c00dd2231077e3a01aebc  groovy-desktop-amd64.iso

sudodus@bionic64 /media/multimed-2/test/ubuntu $ sudo kpartx -av groovy-desktop-amd64.iso
add map loop0p1 (253:0): 0 5725588 linear 7:0 64
add map loop0p2 (253:1): 0 9952 linear 7:0 5725652
add map loop0p3 (253:2): 0 600 linear 7:0 5735604

sudodus@bionic64 /media/multimed-2/test/ubuntu $ lsblk -fm /dev/loop0
NAME      FSTYPE  LABEL              UUID                                 MOUNTPOINT  SIZE OWNER GROUP MODE
loop0     iso9660 Ubuntu 20.10 amd64 2020-10-09-21-15-31-00                           2,8G root  disk  brw-rw----
├─loop0p1 iso9660 Ubuntu 20.10 amd64 2020-10-09-21-15-31-00                           2,7G root  disk  brw-rw----
├─loop0p2 vfat    ESP                6508-0C9E                                        4,9M root  disk  brw-rw----
└─loop0p3                                                                             300K root  disk  brw-rw----

sudodus@bionic64 /media/multimed-2/test/ubuntu $ sudo kpartx -d groovy-desktop-amd64.iso
loop deleted : /dev/loop0

After booting, there is a 'writable' partition.

sudodus@bionic64 /media/multimed-2/test/ubuntu $ lsblk -fm /dev/sdc
NAME   FSTYPE  LABEL              UUID                                 MOUNTPOINT  SIZE OWNER GROUP MODE
sdc    iso9660 Ubuntu 20.10 amd64 2020-10-09-21-15-31-00                          29,1G root  disk  brw-rw----
├─sdc1 iso9660 Ubuntu 20.10 amd64 2020-10-09-21-15-31-00                           2,7G root  disk  brw-rw----
├─sdc2 vfat    ESP                6508-0C9E                                        4,9M root  disk  brw-rw----
├─sdc3 iso9660 Ubuntu 20.10 amd64 2020-10-09-21-15-31-00                           300K root  disk  brw-rw----
└─sdc4 ext4    writable           8b73f753-eb94-40ec-8995-f57c64933d3f            26,3G root  disk  brw-rw----

Let us hope, that it is not only a happy coincidence, but that squashing bug #1886148
did also squash bug #1895329, but now there is another 3rd partition,
and we should look for a 4th partition.