Selectize js submitting only last selected item

I'm currently using Selectize.js with a 'select' item and multiple options. My html is something like:

                    Genre(s):
                    
                         $val): ?>
                            
                                
                            >
                                
                            
                        
                    
                

So far so good. The items are being loaded correctly and until this point everything works great. The issue is that when the form is submitted only the last value selected is the one that is available, so if I selected multiple items I'm getting just one:

["record_genre"]=> string(1) "1"

Here's my JS setup:

$('.multi-selectize').each(function(){
            //console.log($(this))
            $(this).selectize({})

        })

Any suggestions?

Thanks.