http://svn.apache.org/viewvc/tomcat/tc8.5.x/trunk/java/org/apache/coyote/http2/LocalStrings.properties?view=markup#l107 The log message "upgradeHandler.pruneIncomplete" uses {0} twice instead of {0} and {1}. So you can't see how many streams were not pruned. You can see here how two variables are passed in - http://svn.apache.org/viewvc/tomcat/tc8.5.x/trunk/java/org/apache/coyote/http2/Http2UpgradeHandler.java?view=markup#l1016
Thanks for the report. Fixed in 9.0.x for 9.0.0-M10 and 8.5.x for 8.5.5 onwards.