How to reproduce

• create user task in Camunda modeler
• add task listener (confirmed on "complete" event) with Java class implementation $TaskListener
• morph task type to service task, the task listener is no longer visible in the modeler UI properties view
• implement service for this task
• deploy process

When the engine execution completes the the service task, the engine tries to invoke the task listener and throws an ProcessEngineException

org.camunda.bpm.engine.ProcessEngineException: $TaskListener doesn't implement interface org.camunda.bpm.engine.delegate.ExecutionListener