First detected the problem when I change some information in a row in a table.
As soon as you select a input field to edit all the checkboxes that show that one value is set to NULL are cleared.
And even if you only have one value that is NULL, and you make sure to check the checkbox it clears as soon as you click the submit button.
I then tried to insert an new row, but same result.
All the checkboxes cleard.
Have tried on serveral servers, running both MySQL 5.0.x and MySQL 4.1.x. Same problem on every installation.
So, my suggestion is that there is a bug in the code.