save, update, delete not effective with FindFirst() - probably due to multi column primary key

[Krzysiaczek]

I've got a table with three fields a,b,c where a and b are defined as a primary key

CREATE TABLE `ac` (
  `a_id` bigint(20) unsigned NOT NULL,
  `c_id` varchar(20) NOT NULL,
  `c_type` int(11) DEFAULT '0',
  PRIMARY KEY (`a_id`,`c_id`),
  KEY `FK_ac_idx` (`a_id`),
  KEY `FK_ac_cat_idx` (`c_id`),
  KEY `act_cat` (`c_type`),
  CONSTRAINT `FK_ac_cat` FOREIGN KEY (`c_id`) REFERENCES `cats` (`c_id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `FK_ac` FOREIGN KEY (`a_id`) REFERENCES `acts` (`a_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

When I get a record from that table using FindFirst

$ac = Ac::FindFirst("a_id = $aId AND c_type = 0");

I am trying to update the $ac->c_id = "something else" value but I can't $ac->update(), $ac->save() or $ac->delete() it. All methods returns true so no error messages are available but record remains unchanged in the DB.

I've tried to extend the Model definition by pointing out which fields represents the Primary Key but no change in results.

class Ac extends Model
{
    /**
     * @Primary
     * @Column(type='string', length=20, nullable=false)
     */
    public $a_id;
    /**
     * @Primary
     * @Column(type='integer', nullable=false)
     */
    public $c_id;
    public $c_type;

Any ideas how to fix it?

I'm on php 7.4.28 and phalcon 4.1.2

[Krzysiaczek]

After coming back to this I can see that $ac->delete() seems to work with FindFirst so I can remove and recreate instead of update().
Still not sure why update() or save() do not work?