Permission changed with upgrade

Hi!
I’ve been a problem with all files’ permission when I’ve automatically upgrade my site from ver. 2.7.0 to 2.8.
I’ve used the automatic upgrade from Administration panel.
After the upgrade, the homepage and all the site worked well, but I cannot login.
The error was the “classic” “500 Internal server error”, both for wp-login.php file than for wp-config.php.
And also was impossible visualize the index.php file inside the /wordpress folder (but the index.php on the root worked fine).

After a littel trouble, I’ve found all the files’ permission changed, without the “Execute” permission enabled. The permission was “644” instead “755”.
I’ve changed all the permission to “755” (Owner Read, Write and Execute, Group and others Read and Execute) and now WP works well.

It’s “normal”, it’s a bug or i’m particularly unlucky?
Thanks,
a_