I created a more simple test and had success saving a single text field contained in a group. So now I wonder if there's something else going on.
Test Case:
group (id: group)
-text (id: text)
Code:
MB_Custom_Table_API::create( "{$prefix}testing", array(
'group' => 'TEXT NOT NULL'
) );
Setting that group to cloneable (to emulate the design of my production meta box has no affect on saving the data successfully - a good thing!)
I'm continuing to look into this.