Just an update...I eliminated the tiles and spring:theme code, and still get the error by putting the same large content in the page with "<c:out", followed by a "jsp:include". That is, the JSP include doesn't appear in the output unless I set ""<%@ page buffer="256kb" %>.
Again, this is resolved by increasing the buffer size. I've also noticed the response is marked as committed with the large output before the "jsp:include". This doesn't seem to be an issue in Tomcat 9, but does seem to be in Tomcat 10.
