I too have upgraded from 1.3.9g to 1.3.9h and am experiencing the same problem. Original installation was 1.3.9b but I have upgraded religiously since then.

I have added new products and their descriptions as normal, but the description field disappears if I go back and have to edit something.

I have retraced my steps twice now but still the same problem. I noticed the problem occured before I added any of the new files, so assume it was one of the pages that was overwritten.

Server details are as follows:

PHP 5
Charset from english.php: iso-8859-1
SQL Charset: utf8 utf8_general_ci UTF-8 Unicode 3
SQL 5

Hope this helps. If anything isn't clear please ask and I shall clarify.

Thanks