Bootstrap tagsinput and typeahead with ajax

I am using Bootstrap tagsinput and typeahead, which works perfectly when I use it this way:

$(document).ready(function() {
    $('.tagsinput-typeahead').tagsinput({
        typeahead: {
            source: $.getJSON('/ajax/tags.php'),
            afterSelect: function() {
                this.$element[0].value = '';
            }
        }
    });
});

But I want to use it with ajax, but it only shows me the first letter.

$(document).ready(function() {    
    jQuery('.tagsinput-typeahead').tagsinput({
        typeahead : {
            source: function (query, process) {
                return jQuery.get('/ajax/tags.php?query=' + query, function(data){  
                // return process(data);              
                });
            }
        }
    });
});

Even when I return process(data); it won't work.

In addition when I click on one suggestion the letter will appear in the inputfield as well.

My tags.php looks like this:

$json = [];
$json[] = "test";
$json[] = 'Washington';
$json[] = 'London';
$json[] = 'Germany';
$json[] = 'Denmark';
echo json_encode($json); 

Any ideas?