Stationary lava is also a LAVA ignition cause.

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -561,6 +561,7 @@ public class CraftEventFactory {
         IgniteCause cause;
         switch (igniter.getType()) {
             case LAVA:
+            case STATIONARY_LAVA:
                 cause = IgniteCause.LAVA;
                 break;
             case DISPENSER: