We noticed that users are able to right click and paste into the excerpt field without validation on the character limit…

add this to the JQuery solves the problem:

$(EXCERPT_INPUT).mouseout(function() {
				var text = $(EXCERPT_INPUT).val();
				var textLength = text.length;
				if(textLength>excerpt_charLimit){
					alert("Please check your excerpt as it may have been truncated");
				}
				$(EXCERPT_INPUT).keyup();
			});

https://www.ads-software.com/extend/plugins/excerpt-character-limiter/