Index: trunk/test/unit/org/openstreetmap/josm/actions/CreateMultipolygonActionTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/actions/CreateMultipolygonActionTest.java	(revision 17341)
+++ trunk/test/unit/org/openstreetmap/josm/actions/CreateMultipolygonActionTest.java	(revision 17343)
@@ -4,4 +4,5 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -12,6 +13,6 @@
 import java.util.TreeMap;
 
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
-import org.junit.jupiter.api.Test;
 import org.openstreetmap.josm.TestUtils;
 import org.openstreetmap.josm.command.SequenceCommand;
@@ -156,3 +157,20 @@
 
     }
+
+    /**
+     * Non-regression test for <a href="https://josm.openstreetmap.de/ticket/20110">Bug #20110</a>.
+     * @throws Exception if an error occurs
+     */
+    @Test
+    void testTicket20110() throws Exception {
+        DataSet ds = OsmReader.parseDataSet(TestUtils.getRegressionDataStream(20110, "data.osm"), null);
+        assertEquals(1, ds.getRelations().size());
+        Relation mp = ds.getRelations().iterator().next();
+        assertEquals("wetland", mp.get("natural"));
+        long numCoastlineWays = ds.getWays().stream().filter(w -> "coastline".equals(w.get("natural"))).count();
+        Relation modMp = createMultipolygon(ds.getWays(), "type:way", mp, false);
+        assertNotNull(modMp);
+        assertEquals("wetland", modMp.get("natural"));
+        assertEquals(numCoastlineWays, ds.getWays().stream().filter(w -> "coastline".equals(w.get("natural"))).count());
+    }
 }