worldedit-core/src/main/java/com/sk89q/worldedit/function/operation/ForwardExtentCopy.java

@@ -446,6 +446,10 @@ public class ForwardExtentCopy implements Operation {
 
         }
         affectedBlocks += blockCopy.getAffected();
+        if (copyingBiomes) {
+            // We know biomes will have happened unless something else has gone wrong. Just calculate it.
+            affectedBiomeCols += source.fullySupports3DBiomes() ? (getAffected() >> 2) : (region.getWidth() * region.getLength());
+        }
         //FAWE end
         return null;
     }