Michel,
What you say is possible, and has happened. But dumb things happen.
Those dumb things could happen with non site-local addresses as well.
But look. Ultimately I think we as a community do need to own up to
better tooling, which can lead to better expectations. Also, I don't
see any reason why an IP v6 prefix allocation can't linger for a very
long time after a contract ends. The tools need to set expectations,
and perhaps some of the DHCP prefix delegation code can help here.
Regards,
Eliot