Why is update([$data]) on a date field not working?

Hi, I'm using a version of Laravel 7. I have defined a date column on my model's table using timestamp('custom_date') in the migration. I have added it to the $dates field of the model. When creating a new model (using Eloquent), I pass in a unix timestamp and the date is saved fine in the DB. However, when updating using Eloquent's update() function, I pass in the same unix timestamp and the date is saved something weird in the DB like -0001-11-30 00:00:00.

Creation of column (over two migrations unfortunately, forgot to make nullable first time):

Schema::table('model', function (Blueprint $table) {
  $table->timestamp('custom_date');
});

Schema::table('model', function (Blueprint $table) {
        DB::statement("ALTER TABLE model MODIFY COLUMN custom_date TIMESTAMP NULL AFTER content");
        DB::statement("UPDATE model SET custom_date = NULL");
});

Sample $data (works fine for creation and with workaround):

$data = [
	'content' => 'Lorem ipsum',
	'custom_date' => 1600865316
];

Model.php has:

protected $fillable = ['content', 'custom_date'];
protected $dates = ['custom_date'];

Repository functions, with update that doesn't work as expected (for the date only):

public function create($data)
{
	$model = Model::create($data);
	return $model;
}

public function update($id, $data)
{
	// here I verified that if I make a Carbon object from the timestamp, like so-
	// Carbon::createFromTimestamp($data['custom_date']);
	// and dd or codecept_debug it, it is the correct date,
	// though it is saved wrong in the db.

	// Docs states that timestamp should work, and it works for create.

	return Model::where('id', $id)->update($data);
}

I have verified that the timestamp converts to a valid and correct Carbon object. I have found a workaround, which is to set that attribute directly, save() the model instance, and then unset that field before the update().

Workaround:

public function update($id, $data)
{
   $model = Model::find($id);
   $model->custom_date = $data['custom_date'];
   $model->save();
   unset($data['custom_date']);

   return Model::where('id', $id)->update($data);
}

However it seems odd and this behaviour isn't mentioned anywhere in the docs (that I could find). Is this a bug or am I missing something?