diff -r 14e098407bb0 -r ad69fd32778e src/jdk.compiler/share/classes/com/sun/tools/javac/api/JavacTaskPool.java
--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/api/JavacTaskPool.java	Mon Oct 21 15:11:44 2019 +0200
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/api/JavacTaskPool.java	Mon Oct 21 15:38:26 2019 +0200
@@ -52,6 +52,7 @@
 import com.sun.source.util.TaskListener;
 import com.sun.source.util.TreeScanner;
 import com.sun.tools.javac.code.Kinds;
+import com.sun.tools.javac.code.Preview;
 import com.sun.tools.javac.code.Symbol;
 import com.sun.tools.javac.code.Symtab;
 import com.sun.tools.javac.code.Type;
@@ -257,6 +258,8 @@
                 ((ReusableJavaCompiler)ReusableJavaCompiler.instance(this)).clear();
                 Types.instance(this).newRound();
                 Check.instance(this).newRound();
+                Check.instance(this).clear(); //clear mandatory warning handlers
+                Preview.instance(this).clear(); //clear mandatory warning handlers
                 Modules.instance(this).newRound();
                 Annotate.instance(this).newRound();
                 CompileStates.instance(this).clear();