If jvmRoute attribute is not set in server.xml on engine element, ModClusterService generates a jvmRoute (UUID). This UUID is not added in the value of the cookie "jsessionid", so the sticky session doesn't work. When the jvmRoute is defined in the server.xml all works fine.

I known no much about lifecyle in Catalina, but my feeling is that ModClusterService generates the jvmRoute (with JvmRouteFactory)
after the creation of SessionIdGenerator by the org.apache.catalina.Manager. When ModClusterService injects the generated jvmRoute it's too, the SessionIdGenerator is already created.