As already mentioned, in the second table, you are missing namcespace declaration.
Here is the modified version of your workflow where I just added 2 String Manipulaiton nodes to show what the problem is with the second table. In one, I have removed the namespace, and in the other, I have declared it. Both then can be converted to XML.
78954 - String to XML does not support self closing tags.knwf (106.4 KB)
That expalins the missing namespace declaration in the second table then.