jQuery UI autocomplete values not getting selected after selection from mouse

I have created a functionality of autocomplete using jquery. It works perfectly fine but for ex:

Whenever I type a text and and select the values from the list using mouse, it gets selected for the first time but if I again type and select the list. The values dont get selected.

Below is my code. I am not able to get why its not wroking.

$(document).ready(function () {
$('#txtAssignVendor').autocomplete({
    source: AppConfig.PrefixURL + 'VendorData.ashx',
    position: {
        my: "left bottom",
        at: "left top",
    }
});});

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