`df`: mounted ISO image is not shown in list - in contrast to GNU

While testing for issue #6073, I found an unrelated issue when having a ISO image mounted:

GNU:

uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
tmpfs          tmpfs     1.6G  2.3M  1.6G   1% /run
/dev/nvme0n1p7 ext4       97G   33G   59G  36% /
tmpfs          tmpfs     7.7G   21M  7.7G   1% /dev/shm
tmpfs          tmpfs     5.0M  4.0K  5.0M   1% /run/lock
efivarfs       efivarfs  192K  127K   60K  68% /sys/firmware/efi/efivars
/dev/nvme0n1p1 vfat      256M   88M  169M  35% /boot/efi
/dev/nvme0n1p8 ext4      773G  215G  519G  30% /home
tmpfs          tmpfs     1.6G  1.8M  1.6G   1% /run/user/1000
/dev/nvme0n1p5 fuseblk   733G  392G  342G  54% /media/uli/WData
/dev/loop20    iso9660   604M  604M     0 100% /media/uli/OpenBSD_amd64   7.4 Install CD

uutils:

uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ ~/dev_rust/coreutils/target/debug/coreutils df -Th
Filesystem     Type     Size  Used Avail Use% Mounted on
udev           devtmpfs 7.7G     0  7.7G   0% /dev
tmpfs          tmpfs    1.6G  2.3M  1.6G   1% /run
/dev/nvme0n1p7 ext4      97G   33G   59G  36% /
tmpfs          tmpfs    7.7G   21M  7.7G   1% /dev/shm
tmpfs          tmpfs      5M    4K    5M   1% /run/lock
efivarfs       efivarfs 192K  127K   60K  68% /sys/firmware/efi/efivars
/dev/loop0     squashfs 128K  128K     0 100% /snap/bare/5
/dev/loop1     squashfs  56M   56M     0 100% /snap/core18/2796
/dev/loop2     squashfs  56M   56M     0 100% /snap/core18/2812
/dev/loop3     squashfs  64M   64M     0 100% /snap/core20/2105
/dev/loop4     squashfs  64M   64M     0 100% /snap/core20/2182
/dev/loop5     squashfs  75M   75M     0 100% /snap/core22/1033
/dev/loop6     squashfs  75M   75M     0 100% /snap/core22/1122
/dev/loop7     squashfs 165M  165M     0 100% /snap/gnome-3-28-1804/198
/dev/loop8     squashfs 350M  350M     0 100% /snap/gnome-3-38-2004/140
/dev/loop9     squashfs 165M  165M     0 100% /snap/gnome-3-28-1804/194
/dev/loop10    squashfs 350M  350M     0 100% /snap/gnome-3-38-2004/143
/dev/loop11    squashfs 497M  497M     0 100% /snap/gnome-42-2204/132
/dev/loop12    squashfs 497M  497M     0 100% /snap/gnome-42-2204/141
/dev/loop13    squashfs  82M   82M     0 100% /snap/gtk-common-themes/1534
/dev/loop14    squashfs  92M   92M     0 100% /snap/gtk-common-themes/1535
/dev/loop15    squashfs 259M  259M     0 100% /snap/qt551/43
/dev/loop16    squashfs 302M  302M     0 100% /snap/qt5-core20/16
/dev/loop17    squashfs 302M  302M     0 100% /snap/qt5-core20/17
/dev/loop19    squashfs 259M  259M     0 100% /snap/qt551/44
/dev/loop18    squashfs 219M  219M     0 100% /snap/qt5-core22/13
/dev/loop21    squashfs 220M  220M     0 100% /snap/qt5-core22/14
/dev/loop22    squashfs  13M   13M     0 100% /snap/snap-store/959
/dev/loop23    squashfs  41M   41M     0 100% /snap/snapd/20671
/dev/loop24    squashfs  40M   40M     0 100% /snap/snapd/21184
/dev/loop25    squashfs 4.8M  4.8M     0 100% /snap/wonderwall/35
/dev/loop26    squashfs 2.8M  2.8M     0 100% /snap/wonderwall/39
/dev/nvme0n1p1 vfat     256M   88M  169M  35% /boot/efi
/dev/nvme0n1p8 ext4     773G  215G  519G  30% /home
/dev/loop27    squashfs  13M   13M     0 100% /snap/snap-store/1113
tmpfs          tmpfs    1.6G  1.8M  1.6G   1% /run/user/1000
/dev/nvme0n1p5 fuseblk  733G  392G  342G  54% /media/uli/WData
uli@hp13-ulix:/media/uli/OpenBSD_amd64   7.4 Install CD$ 

The ISO image mount doesn't show up in the list of uutils.
In contrast, it shows all the /snap/ mounts instead.

Start at the df command entry point and reproduce the comparison with an ISO image mounted alongside the listed /snap/ mounts. Investigate why the ISO mount is omitted, then verify that it appears in the output while the existing mounts remain listed.