Hi Sajin,
Thanks for contacting syncfusion support.
We have analyzed the reported case and found that it is the correct behavior.While enabling AutoComplete mode as Suggest and type a letter in Single Selection mode, it will clear the old items in listview and suggest the items based on the typed value. In that time, as per the behavior SelectedIndex will be updated based on the new items collection. If you need a original index of that item, you can get it from the original items collection.
Please let us know if you have anyother concerns.
Regards,
Bharathi R