Are you talking about this error call out that appears when you put your mouse pointer over the error cross mark ? ( Find attached ).
I can see the same behavior at my end where it is restricting it to 512 characters. I am not totally sure the underlying rule that is blocking it but from user experience prospective, i think it is good to have such limitations. As you might have noticed, the call out vanished automatically after few seconds, so with the limited time , it is difficult for user to read more than 400 characters. Also, it does not look good to show such big error message in form of call outs.
A better alternative could be to display them as page level error message on top of harness.