When a widget is linked to a reference table, front edition use a 'AmetysFrontEdition.widget.TagWidget'

The getValue is changed to this code :

getValue: function() {
    // tag field is now always return multiple values, but that's not fine for ours widgets
    var value = this.callParent(arguments);
    return multiple ? value : (value && value.length > 0 ? value[0] : null);
}

The problem with this is that the value can be a String, and the length will then be the length of the string.

So only the 1st character will be sent