Thank you very much, eber42, for testing the new packages, and for posting such a detailed bug report.

You are correct, it should NOT unmount like that. That code was added to allow users to run a second image instead of the first image, but it is unmounting even when the image file is the same.

I will fix that bug ASAP (this evening) and post the fix.

As always, if you need a working version, the version in Extras should do the trick. But I would prefer you to keep beta testing...

UPDATE: It's a dumb bug. I wrote "echo /debian/var/lock/chroot-complete" instead of "cat /debian/var/lock/chroot-complete". You'll see a fix when I get home.