Update an existing pivot record

If you want to update an existing pivot record on the table, use updateExistingPivot instead of syncWithPivotValues.

// Migrations
Schema::create('role_user', function ($table) {
    $table->unsignedId('user_id');
    $table->unsignedId('role_id');
    $table->timestamp('assigned_at');
})
 
// first param for the record id
// second param for the pivot records
$user->roles()->updateExistingPivot(
    $id, ['assigned_at' => now()],
);

Tip given by @sky_0xs