What are the steps to reproduce your problem?
1. Setup spring boot project (you can use the one in the attachments) using

• camunda-bpm-spring-boot-starter-webapp
• camunda-engine-rest

2. Run the project
3. Execute the following query:

/POST http://localhost:8080/rest/engine/default/external-task/fetchAndLock
{
	"asyncResponseTimeout": 10000,
	"workerId": "worker",
	"maxTasks": 1,
	"usePriority": true,
	"topics": [{
		"topicName": "topic",
		"lockDuration": 50000
	}]
}

What is the problem?
The asyncResponseTimeout is not taken into account and the response [] is returned immediately

Note:
The problem is that the projects uses camunda-bpm-spring-boot-starter-webapp which contains dependency to camunda-engine-rest which does not support long polling and it is also mapped to the same endpoint.