I am on Splunk 7.1.4 and there is some issue with the way multi select is behaving.
After choosing a value from the auto populated list. The list is not hiding until clicked somewhere else.
Earlier the list used to go away once a value is chosen into.
That is the intended behaviour of a MULTI select Input that you can select more than one option.
If you want to select only a SINGLE value you should take a Dropdown Input.