Web app settings gets overwritten and deleted

When having appsettings defined in arm templates for web app, no matter through a **siteConfig** object or define a **Microsoft.Web/sites/config** child resource, it always overwrite all existing appsettings.

For example, I have an existing setting in cloud which is `environment : dev`, and then in my arm template, I only defined a setting `databaseKey : xxxxx`. If I deploy the arm template again, with Incremental mode set, when it finishes, I expect the app settings have both settings, which should be
```
environment : dev
databaseKey : xxxxx
```
However, it complete deletes `environment : dev` and only kept `databaseKey : xxxxx`. Is there anyways to make sure that existing settings don't get deleted?
I've seen serveral people encountered this problem online and have not found a proper solution to that. A suggestion is to deploy appsettings through cli or powershell after arm template is done. But that really make things complicated and prone to error.

The issue names ARM templates, siteConfig, and a Microsoft.Web/sites/config child resource but no repository file or test. Start by reproducing an incremental deployment with an existing environment setting and a newly declared databaseKey, then verify whether both settings remain afterward; done means existing app settings are preserved.

Written by the indexing model from the issue text.