apache group ownership inconsistencies on Verify

I'm running 6.x.-0.3-rc2 installed using install.sh on an Ubuntu 9.04/Apache2/PHP5/MYSQL5 stack.

I untarred an existing site into /var/aegir/platforms/example.
I ran 'chown -R aegir:aegir example' to ensure all files had correct ownership.

I imported the platform into aegir and the site. It verified okay.

Later I wanted to re-verify the site. This time it failed with the following errors

Could not change permissions sites/example.com to 493 (chmod to 755 failed on sites/example.com)
Could not change permissions sites/example.com/files to 1528 (chmod to 2770 failed on sites/example.com/files)

On checking i found that the ownership of these directories, and the settings.php file had been changed to be aegir:www-data.

My aegir user is a member of the www-data group. the site itself loads fine in a browser whether the group is set as aegir or www-data.

Running the following as root solves the problem of verifying in aegir
chown -R aegir:aegir sites/example.com

The site then verifies again. But resets the ownership to aegir:www-data - so it doesn't verify after that.

This doesn't seem to be a problem all the time. it happened on one other site i imported, but not others.