.img file gets opened as read-only

I'm trying to create an ext4 filesystem in a .img file so that I can transfer it to other machines. Unfortunately, when I unmount and re-mount it, it gets mounted as read-only. I can reproduce the issue on two separate PCs -- a laptop running Ubuntu 20.04 and a desktop running Ubuntu 23.04 -- using the following steps:

1. In GNOME, run Disks, select "New disk image", create a 1GB .img file in your home directory, click "Attach new image", then "Format partition" and format it as ext4. This creates device /dev/loop5, contents mounted at /media/my-username/volume-label.
2. Open it in GNOME Files (nautilus). Copy some files into it.
3. Unmount it using GNOME Files (nautilus).
4. Mount it again by clicking on the .img file in GNOME Files.
5. Try to copy some more files - you can't - it's read-only.

How can I mount it multiple times properly?

P.S. In case it's relevant, a puzzling thing is when I created the file system, a window opened very briefly asking for my password, but it closed before I had a chance to type anything, and then the new FS opened in Files seemingly without any issues.

Further info:

$ ls -l *.img;id
-rw-rw-r-- 1 k314159 k314159 1000000000 Aug 16 10:27 t.img
uid=1000(k314159) gid=1000(k314159) groups=1000(k314159),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),132(lxd),133(sambashare),136(docker)
$ grep loop5 /etc/mtab
/dev/loop5 /media/k314159/test ext4 rw,nosuid,nodev,relatime 0 0
# Then after using GNOME Files (nautilus) to dismount & remount:
$ grep loop5 /etc/mtab
/dev/loop5 /media/k314159/test ext4 ro,nosuid,nodev,relatime 0 0

I found that the issue is that the GNOME Disk Image Mounter, when run from nautilus, always mounts .img files in read-only mode. There is a request to add a read-write option but this has not been implemented yet:

https://gitlab.gnome.org/GNOME/gnome-disk-utility/-/issues/126

The workaround is to start the Disks GUI and mount the .img from there, and untick the "mount read-only" check box at the bottom. Alternatively, run the disk image mounter from the command line:

gnome-disk-image-mounter --writable <container path>