RPM deletes required files if -U -prefix is used together

Hello

there seams to be a problem if the Upgrade option is used on
relocatable packages. If i do the following:

rpm -i mypack-1-01.i586.rpm --prefix=/myprefix

and then upgrade to a new version of mypack like this:

rpm -U mypack-2-05.i586.rpm --prefix=/myprefix

Bad things happen. The directory /myprefix only contains the files that
were added to the new version. All files that exist in both versions of
the rpm are missing.

As far as I understand the -U option. It is just a rpm -i followed by a
rpm -e on the old version. But the -e will only delete files that don't
belong to the new version of the package. I guess this check is broken
if used with relocatable packages.

Is there a workaround for this problem. Anybody noticed this ? Could
somebody give me a hint where in the rpm code I should look for the
bug. Is this check done in the rpmlib or in some scripts ? Any hints
are welcome.

I'm using rpm 4.1.1

Thanks
Berni.