Comment 3 for bug 1277146

It seems to be not a bug,

according to the answers to that question linked in the previous post.

The iso file is not mounted read only - even if the iso image file may be read only.
That allows for changes in the metadata of the iso file - even if the files in the iso filesystem can not be changed.

Adding the mount option "ro" to explicitly mount read only prevents the changes of the iso or udf image file, solving the problem:

sudo mount -o ro,loop testimage.iso /mnt