Present on both Unity versions 2020.1.0b6 & 2019.3.0f3. I did not test other versions.

EDIT: Confirmed. Dup of https://issuetracker.unity3d.com/issues/rendertexture-is-flipped-upside-down-when-using-rendertexturedescriptor

Given this scene...

...with a camera at the center.

Rendering into a RenderTexture created like this:

_renderTexture = new RenderTexture(512, 512, 1,  GraphicsFormat.R32G32_SFloat);
_renderTexture.useMipMap = false;
_renderTexture.autoGenerateMips = false;
_renderTexture.useDynamicScale = false;
_renderTexture.dimension = TextureDimension.Cube;
_renderTexture.filterMode = FilterMode.Point;

Correctly results in this:

Note: the color change is due to the R32G32_SFloat format. That format is required for our project's usecase so I have left it as is

However when making (what appears to be) a RenderTexture with the same settings via a RenderTextureDescriptor...

var descriptor = new RenderTextureDescriptor();
descriptor.dimension = UnityEngine.Rendering.TextureDimension.Cube;
descriptor.width = 512;
descriptor.height = 512;
descriptor.volumeDepth = 1;
descriptor.depthBufferBits = 16; // set so it matches Approach2
descriptor.stencilFormat = GraphicsFormat.None; // set so it matches Approach2
descriptor.bindMS = false;
descriptor.msaaSamples = 1; // set as required
descriptor.graphicsFormat = GraphicsFormat.R32G32_SFloat;
descriptor.useMipMap = false;
descriptor.autoGenerateMips = false;
_renderTextureFromDescriptor = new RenderTexture(descriptor);
_renderTextureFromDescriptor.filterMode = FilterMode.Point;

...this is the result:

The top and bottom faces of the cubemap appear flipped to me. Here is an image I've scribbled on to show the face edge and point of interest

The scene in this test case shows the issue.

• Open this project in the Unity Editor
• Click the Play button
• Once running, navigate to the object named 'Test' in the inspector
• You will see two exposed RenderTexture fields. Double clicking each one will show the cubemap that has been captured by this test-case
• Render Texture shows the result when the RenderTexture was created without using a RenderTextureDescriptor
• Render Texture From Descriptor shows the result when the RenderTexture was created using a RenderTextureDescriptor

The Monobehaviour for the 'Test' object can be found here Assets/Test.cs