That doesn't sound correct. I can't imagine a heap dump being that small. I would bet that Apps Manager is unable to talk to the corresponding Spring Boot Actuator endpoint which generates the heap dump. Either that, or the Actuator endpoint is responding with nothing.
I would suggest looking in the Javascript Console & Network tabs of your Browsers Dev Tools when you make the request. See if there are any errors or 4xx/5xx responses from your app. Also, look at your app's logs to see if a request comes in for Actuator to trigger the heap dump & what response code & response size was reported.
Hope that helps!