This bug takes me lot of time to find it, the problem is the actually fix with the set_relation. Don't worry about those kind of problems as I am preparing a big release as for the next version and I will probably rewrite the set_relation queries as I have some small problems with the callback_column callbacks and set_relation.
Ok back to your problem now. The problem is that you probably have some conflict with the field name and the set_relation . For example if you have the 'username' at your basic table and you have a set_relation like this:
$this->grocery_crud->set_relation('user','cms_users','username'); //same field name
With the last bug fix you will have a problem using the callback as you have to do it like this (let's say that your primary table is "users"):
$this->grocery_crud->callback_column('users.username', function($value, $row) {
return uc_first($value);
});
and it will work.