I've solved the bug changing the notification-daemon-0.3.4/debian/patches/01_ubuntu_theme.diff file, replacing
+#define WIDTH 300
by
+#define WIDTH 500
After rebuilding and reinstalling the new package, the notification bubbles appears always perfectly, both the update-notifier's bubble and the "low disk space"'s bubble.
I've solved the bug changing the notification- daemon- 0.3.4/debian/ patches/ 01_ubuntu_ theme.diff file, replacing
+#define WIDTH 300
by
+#define WIDTH 500
After rebuilding and reinstalling the new package, the notification bubbles appears always perfectly, both the update-notifier's bubble and the "low disk space"'s bubble.