The API EncodeNativeArrayToJPG needs a parameter `rowBytes` passed (5th parameter) before a custom quality can be passed (6th parameter). If `rowBytes` is not know, you can pass in zero. So the issue is resolved by using
`EncodeNativeArrayToJPG(buffer, rtFormat, (uint)w, (uint)h, 0, 75);`
See the documentation on how to use EncodeNativeArrayToJPG :
https://docs.unity3d.com/6000.0/Documentation/ScriptReference/ImageConversion.EncodeNativeArrayToJPG.html