function setupAutoCompleteValidation() {
  // Validate autocomplete-validate
  $("div.harnessContent").on("change",".autocomplete-validate input", function (event) {
    var input = pega.util.Event.getTarget(event);
    var $input = $(input);  
        
    // Trim the input value
    var value = _.trim($input.val());
    $input.val(value);
    
    // If the input value is blank, no need to proceed. Let the OOTB events takeover.
    if ($input.val() === "") {
      return;
    }
    
    // Do not fire this validation when the user triggers onblur by clicking on the autocomplete list
    var $acDIV = $("div#po0");
    if ($acDIV.css("visibility") === "visible" && $acDIV.is(':hover')) {
  return;
    }
    
    var attr = $input.attr("data-attributes");
    var obj = pega.lang.JSON.parse(attr);
    var valueFound = false;
    $("div#po0 table[param_name='"+obj[0]+"'] table.gridTable[grid_ref_page='"+obj[4]+"'] tr[data-gargs] td:first-child div.cellIn > span:first-child")
      .each(function(key, elem) {        
       var $elem = $(elem);
       if (_.trim($elem.text()) === value) {
          valueFound = true;
        }
      });
    if (!valueFound) {      
      event.preventDefault();
      event.stopPropagation();      
      window.setTimeout(function() {
        // It is possible for the original input Element to have changed because the section was refreshed.
        // Find it again in the current harnessContent
        $input = $("div.harnessContent").find("input[name='"+$input.attr("name")+"']");
        
        var errorMessage = "'"+value+"' is invalid";
        validation_displayFieldError([new validation_Error($input[0], errorMessage)],[],true);   
        $input.val("");
        $input.focus();
      },800)      
    } 
  });  
}

$(document).ready(function(){ 
  setupAutoCompleteValidation();
});

The script above takes advantage of the fact that when the user change the input value of an autocomplete control, the OOTB behavior is that it fetches the list of values applicable for the supplied input and puts them in a DIV element that can be identified by the div selector div#po0.

To use the above, you need to add

autocomplete-validate

in the Cell read-write classes property of your cell using the Autocomplete control (see attached jpg file if its not clear).  

The script above can only validate user inputs that will trigger the input "change" event.

The downside of the above is that if the way the autocomplete values are sent to the front-end changes (e.g., Pega restructured the contents of div#po0) the above will no longer work.

The product should really provide an option to toggle on/off autocomplete value client validation because this is a very common and widely used requirement.