What happens behind the file system when moving the source of a loop drive into its mountpoint?

Question

I moved the source of a loop drive into its mountpoint and umount it. Then the source disappeared.

Note that after moving the source to its mountpoint (or other location), the source was shown as deleted:

$ losetup --all
/dev/loop0: []: (/mnt/storage/test.iso (deleted))

and the space the source previously occupied wasn’t released when I checked via df -h:

# Original Used Space
$ df -h|grep storage
Filesystem                    Type      Size  Used Avail Use% Mounted on
/dev/mapper/vgtank-lvtest     ext4      100M  128K  100M   1% /mnt/storage

# After Creating the File and Mounting
$ dd if=/dev/zero of=/mnt/storage/test.iso bs=64M count=1
1+0 records in
1+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 0.0405464 s, 1.7 GB/s
$ losetup -f test.iso
$ mkdir test
$ mount /dev/loop0 test
$ df -h|grep storage
Filesystem                    Type      Size  Used Avail Use% Mounted on
/dev/mapper/vgtank-lvtest     ext4      100M   64M   36M  64% /mnt/storage

# After Moving the File and Umounting
$ mv test.iso test
$ umount test
$ df -h|grep storage
Filesystem                    Type      Size  Used Avail Use% Mounted on
/dev/mapper/vgtank-lvtest     ext4      100M   64M   36M  64% /mnt/storage

I have two questions here:

1. why can we mv a file to another file system when the file is in used (by losetup)?
2. where did the file go after umounting?