Azure web app V3P3V3 plan manual scaling

Hi people! I have a problem with my web app running on tomcat server on linux. Web app using Premium v3 P3V3. In mu code I am setting manually instances number using AzureResourceManager library and making request like this:

 AppServicePlan appServicePlan.update()
                    .withCapacity(targetInstanceCount)
                    .apply();

In azure Portal I have Scale out method: Manual enabled. Scaling up or down is executed without problem. However every time I scale up programmatically, 3-5 minutes later Azure scales it down. I have noticed just once the instance count was "holding" for long time. But then it has gone. Any idea why Azure interferes and how fix it. And I do not have any configurations for rule based or auto scaling.