Update and There is no data to update.

I can't figure out where the problem is.
I set the model parameters

PHP Code:

protected $table         = 'myTable';

protected $primaryKey    = 'id';

protected $returnType    = 'object';

protected $allowedFields = [
    'field_a',
    'field_b',
    'filed_c',
    'filed_d',
    'field_e',
]; 


Now, inside a method

PHP Code:

$data_update = [
                'filed_a' => '',
                'field_b' => 0,
                'field_c' => date('Y-m-d H:i:s')
               ];

$this->where('id', $row->id)->update($data_update); // <= There is no data to update.

$this->set($data_update)->where('id', $row->id)->update(); // <= Works 


I don't do both updates, both are written to show the two types of approach

$row->id has a value and is correct
What goes wrong when I don't use set()

I've used update() thousands of times, but I don't understand what's going on here

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